Revert at90usb1286 conditional (#19733)

Marlin/src/HAL/AVR/HAL.h

 #include "watchdog.h"
 #include "math.h"
 
-#ifdef IS_AT90USB
+#ifdef USBCON
   #include <HardwareSerial.h>
 #else
   #define HardwareSerial_h // Hack to prevent HardwareSerial.h header inclusion
 //extern uint8_t MCUSR;
 
 // Serial ports
-#ifdef IS_AT90USB
+#ifdef USBCON
   #define MYSERIAL0 TERN(BLUETOOTH, bluetoothSerial, Serial)
 #else
   #if !WITHIN(SERIAL_PORT, -1, 3)

Marlin/src/HAL/AVR/MarlinSerial.cpp

 
 #include "../../inc/MarlinConfig.h"
 
-#if !IS_AT90USB && (defined(UBRRH) || defined(UBRR0H) || defined(UBRR1H) || defined(UBRR2H) || defined(UBRR3H))
+#if !defined(USBCON) && (defined(UBRRH) || defined(UBRR0H) || defined(UBRR1H) || defined(UBRR2H) || defined(UBRR3H))
 
 #include "MarlinSerial.h"
 #include "../../MarlinCore.h"
 
 #endif
 
-#endif // !IS_AT90USB && (UBRRH || UBRR0H || UBRR1H || UBRR2H || UBRR3H)
+#endif // !USBCON && (UBRRH || UBRR0H || UBRR1H || UBRR2H || UBRR3H)
 
 // For AT90USB targets use the UART for BT interfacing
-#if BOTH(IS_AT90USB, BLUETOOTH)
+#if defined(USBCON) && ENABLED(BLUETOOTH)
   HardwareSerial bluetoothSerial;
 #endif
 

Marlin/src/HAL/AVR/MarlinSerial.h

 #endif
 
 // Use the UART for Bluetooth in AT90USB configurations
-#if BOTH(IS_AT90USB, BLUETOOTH)
+#if defined(USBCON) && ENABLED(BLUETOOTH)
   extern HardwareSerial bluetoothSerial;
 #endif

Marlin/src/gcode/control/M111.cpp

   }
   else {
     SERIAL_ECHOPGM(STR_DEBUG_OFF);
-    #if !IS_AT90USB
+    #if !defined(__AVR__) || !defined(USBCON)
       #if ENABLED(SERIAL_STATS_RX_BUFFER_OVERRUNS)
         SERIAL_ECHOPAIR("\nBuffer Overruns: ", MYSERIAL0.buffer_overruns());
       #endif
       #if ENABLED(SERIAL_STATS_MAX_RX_QUEUED)
         SERIAL_ECHOPAIR("\nMax RX Queue Size: ", MYSERIAL0.rxMaxEnqueued());
       #endif
-    #endif // !IS_AT90USB
+    #endif // !__AVR__ || !USBCON
   }
   SERIAL_EOL();
 }

Marlin/src/gcode/queue.cpp

         card.closefile();
         SERIAL_ECHOLNPGM(STR_FILE_SAVED);
 
-        #if !IS_AT90USB
+        #if !defined(__AVR__) || !defined(USBCON)
           #if ENABLED(SERIAL_STATS_DROPPED_RX)
             SERIAL_ECHOLNPAIR("Dropped bytes: ", MYSERIAL0.dropped());
           #endif

Marlin/src/inc/Conditionals_adv.h

   #define NEEDS_HARDWARE_PWM 1
 #endif
 
-#if defined(__AVR__) && defined(USBCON)
-  #define IS_AT90USB 1
-  #undef SERIAL_XON_XOFF // Not supported on USB-native devices
-#else
+#if !defined(__AVR__) || !defined(USBCON)
   // Define constants and variables for buffering serial data.
   // Use only 0 or powers of 2 greater than 1
   // : [0, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, ...]
   #ifndef TX_BUFFER_SIZE
     #define TX_BUFFER_SIZE 32
   #endif
+#else
+  // SERIAL_XON_XOFF not supported on USB-native devices
+  #undef SERIAL_XON_XOFF
 #endif
 
 #if ENABLED(HOST_ACTION_COMMANDS)

Marlin/src/inc/SanityCheck.h

 /**
  * Serial
  */
-#if !IS_AT90USB
+#if !(defined(__AVR__) && defined(USBCON))
   #if ENABLED(SERIAL_XON_XOFF) && RX_BUFFER_SIZE < 1024
     #error "SERIAL_XON_XOFF requires RX_BUFFER_SIZE >= 1024 for reliable transfers without drops."
   #elif RX_BUFFER_SIZE && (RX_BUFFER_SIZE < 2 || !IS_POWER_OF_2(RX_BUFFER_SIZE))
 #endif
 
 /**
- * Emergency Command Parser
+ * emergency-command parser
  */
-#if BOTH(IS_AT90USB, EMERGENCY_PARSER)
+#if ENABLED(EMERGENCY_PARSER) && defined(__AVR__) && defined(USBCON)
   #error "EMERGENCY_PARSER does not work on boards with AT90USB processors (USBCON)."
 #endif