🐛 Fix env validation for 1280/2560 boards (#22150)

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

Marlin/src/pins/mega/env_validate.h

  */
 #pragma once
 
-#if ENABLED(ALLOW_MEGA1280) && NOT_TARGET(__AVR_ATmega1280__, __AVR_ATmega2560__)
-  #error "Oops! Select 'Arduino/Genuino Mega or Mega 2560 or 1280' in 'Tools > Board.'"
-#elif NOT_TARGET(__AVR_ATmega2560__)
-  #error "Oops! Select 'Arduino/Genuino Mega or Mega 2560' in 'Tools > Board.'"
+#if NOT_TARGET(__AVR_ATmega2560__)
+  #if DISABLED(ALLOW_MEGA1280)
+    #error "Oops! Select 'Arduino/Genuino Mega or Mega 2560' in 'Tools > Board.'"
+  #elif NOT_TARGET(__AVR_ATmega1280__)
+    #error "Oops! Select 'Arduino/Genuino Mega or Mega 2560 or 1280' in 'Tools > Board.'"
+  #endif
 #endif
 
 #undef ALLOW_MEGA1280