Disable SERIAL_PORT_2 by default and sanity-check it (#10748)

Marlin/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/default/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/AlephObjects/TAZ4/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/AliExpress/CL-260/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Anet/A6/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Anet/A8/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Azteeg/X5GT/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/BIBO/TouchX/cyclops/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/BIBO/TouchX/default/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/BQ/Hephestos/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/BQ/Hephestos_2/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/BQ/WITBOX/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Cartesio/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Creality/CR-10/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Creality/CR-10S/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Creality/CR-10mini/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Creality/CR-8/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Creality/Ender-2/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Creality/Ender-3/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Creality/Ender-4/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Felix/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Felix/DUAL/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/FolgerTech/i3-2020/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Geeetech/GT2560/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Geeetech/I3_Pro_X-GT2560/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Geeetech/Prusa i3 Pro B/bltouch/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Geeetech/Prusa i3 Pro B/noprobe/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Geeetech/Prusa i3 Pro C/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Geeetech/Prusa i3 Pro W/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Infitary/i3-M508/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/JGAurora/A5/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/MakerParts/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 0
+//#define SERIAL_PORT_2 0
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Malyan/M150/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Malyan/M200/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Micromake/C1/basic/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Micromake/C1/enhanced/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Mks/Sbase/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/RepRapPro/Huxley/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/RepRapWorld/Megatronics/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/RigidBot/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/SCARA/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/STM32F10/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/STM32F4/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-//#define SERIAL_PORT_2 -1
+////#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Sanguinololu/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/TheBorg/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 5
+//#define SERIAL_PORT_2 5
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/TinyBoy2/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Tronxy/X1/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Tronxy/X5S/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Tronxy/XY100/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/UltiMachine/Archim2/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Velleman/K8200/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Velleman/K8400/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Velleman/K8400/Dual-head/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/Wanhao/Duplicator 6/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/adafruit/ST7565/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/delta/FLSUN/auto_calibrate/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/delta/FLSUN/kossel/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/delta/FLSUN/kossel_mini/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/delta/Hatchbox_Alpha/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/delta/generic/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/delta/kossel_mini/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/delta/kossel_pro/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/delta/kossel_xl/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/gCreate/gMax1.5+/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/makibox/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/stm32f103ret6/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/tvrrug/Round2/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/config/examples/wt150/Configuration.h

  *
  * :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
  */
-#define SERIAL_PORT_2 -1
+//#define SERIAL_PORT_2 -1
 
 /**
  * This setting determines the communication speed of the printer.

Marlin/src/inc/SanityCheck.h

   #error "Set SERIAL_PORT to the port on your board. Usually this is 0."
 #endif
 
+#if SERIAL_PORT_2 && NUM_SERIAL < 2
+  #error "SERIAL_PORT_2 is not supported for your MOTHERBOARD. Disable it to continue."
+#endif
+
 /**
  * Dual Stepper Drivers
  */