Add USB serial support to SERIAL_PORT_2 on DUE (#17245)

Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

Marlin/src/HAL/DUE/HAL.h

   #endif
 #endif
 
-
 #include "MarlinSerial.h"
 #include "MarlinSerialUSB.h"
 

Marlin/src/HAL/DUE/MarlinSerial.cpp

 
 // If not using the USB port as serial port
 #if SERIAL_PORT >= 0
-
-  // Preinstantiate
-  template class MarlinSerial<MarlinSerialCfg<SERIAL_PORT>>;
-
-  // Instantiate
-  MarlinSerial<MarlinSerialCfg<SERIAL_PORT>> customizedSerial1;
-
+  template class MarlinSerial<MarlinSerialCfg<SERIAL_PORT>>;      // Define
+  MarlinSerial<MarlinSerialCfg<SERIAL_PORT>> customizedSerial1;   // Instantiate
 #endif
 
-#ifdef SERIAL_PORT_2
-
-  // Preinstantiate
-  template class MarlinSerial<MarlinSerialCfg<SERIAL_PORT_2>>;
-
-  // Instantiate
-  MarlinSerial<MarlinSerialCfg<SERIAL_PORT_2>> customizedSerial2;
-
+#if defined(SERIAL_PORT_2) && SERIAL_PORT_2 >= 0
+  template class MarlinSerial<MarlinSerialCfg<SERIAL_PORT_2>>;    // Define
+  MarlinSerial<MarlinSerialCfg<SERIAL_PORT_2>> customizedSerial2; // Instantiate
 #endif
 
 #endif // ARDUINO_ARCH_SAM

Marlin/src/HAL/DUE/MarlinSerial.h

 };
 
 #if SERIAL_PORT >= 0
-
   extern MarlinSerial<MarlinSerialCfg<SERIAL_PORT>> customizedSerial1;
+#endif
 
-#endif // SERIAL_PORT >= 0
-
-#ifdef SERIAL_PORT_2
-
+#if defined(SERIAL_PORT_2) && SERIAL_PORT_2 >= 0
   extern MarlinSerial<MarlinSerialCfg<SERIAL_PORT_2>> customizedSerial2;
-
 #endif

Marlin/src/HAL/DUE/MarlinSerialUSB.cpp

 
 #include "../../inc/MarlinConfig.h"
 
-#if SERIAL_PORT == -1
+#if HAS_USB_SERIAL
 
 #include "MarlinSerialUSB.h"
 
 }
 
 // Preinstantiate
-MarlinSerialUSB customizedSerial1;
-
-#endif // SERIAL_PORT == -1
+#if SERIAL_PORT == -1
+  MarlinSerialUSB customizedSerial1;
+#endif
+#if SERIAL_PORT_2 == -1
+  MarlinSerialUSB customizedSerial2;
+#endif
 
+#endif // HAS_USB_SERIAL
 #endif // ARDUINO_ARCH_SAM

Marlin/src/HAL/DUE/MarlinSerialUSB.h

 
 #include "../../inc/MarlinConfig.h"
 
-#if SERIAL_PORT == -1
+#if HAS_USB_SERIAL
 
 #include <WString.h>
 
   static void printFloat(double, uint8_t);
 };
 
-extern MarlinSerialUSB customizedSerial1;
+#if SERIAL_PORT == -1
+  extern MarlinSerialUSB customizedSerial1;
+#endif
+
+#if SERIAL_PORT_2 == -1
+  extern MarlinSerialUSB customizedSerial2;
+#endif
 
-#endif // SERIAL_PORT == -1
+#endif // HAS_USB_SERIAL

Marlin/src/inc/Conditionals_LCD.h

   #define SPI_SPEED SPI_FULL_SPEED
 #endif
 
+#if SERIAL_PORT == -1 || SERIAL_PORT_2 == -1
+  #define HAS_USB_SERIAL 1
+#endif
+
 /**
  * This setting is also used by M109 when trying to calculate
  * a ballpark safe margin to prevent wait-forever situation.