Apply env:mega2560ext to relevant boards (#19624)

Marlin/src/pins/pins.h

 #elif MB(AZTEEG_X3_PRO)
   #include "ramps/pins_AZTEEG_X3_PRO.h"         // ATmega2560                             env:mega2560
 #elif MB(ULTIMAIN_2)
-  #include "ramps/pins_ULTIMAIN_2.h"            // ATmega2560                             env:mega2560
+  #include "ramps/pins_ULTIMAIN_2.h"            // ATmega2560                             env:mega2560ext
 #elif MB(FORMBOT_RAPTOR)
   #include "ramps/pins_FORMBOT_RAPTOR.h"        // ATmega2560                             env:mega2560
 #elif MB(FORMBOT_RAPTOR2)
 #elif MB(RL200)
   #include "ramps/pins_RL200.h"                 // ATmega2560                             env:mega2560
 #elif MB(BQ_ZUM_MEGA_3D)
-  #include "ramps/pins_BQ_ZUM_MEGA_3D.h"        // ATmega2560                             env:mega2560
+  #include "ramps/pins_BQ_ZUM_MEGA_3D.h"        // ATmega2560                             env:mega2560ext
 #elif MB(MAKEBOARD_MINI)
   #include "ramps/pins_MAKEBOARD_MINI.h"        // ATmega2560                             env:mega2560
 #elif MB(TRIGORILLA_13)
 #elif MB(CNCONTROLS_15)
   #include "mega/pins_CNCONTROLS_15.h"          // ATmega1280, ATmega2560                 env:mega1280 env:mega2560
 #elif MB(MIGHTYBOARD_REVE)
-  #include "mega/pins_MIGHTYBOARD_REVE.h"       // ATmega1280, ATmega2560                 env:mega1280 env:mega2560
+  #include "mega/pins_MIGHTYBOARD_REVE.h"       // ATmega1280, ATmega2560                 env:mega1280 env:mega2560ext
 #elif MB(CHEAPTRONIC)
   #include "mega/pins_CHEAPTRONIC.h"            // ATmega2560                             env:mega2560
 #elif MB(CHEAPTRONIC_V2)
 #elif MB(GT2560_V3_A20)
   #include "mega/pins_GT2560_V3_A20.h"          // ATmega2560                             env:mega2560
 #elif MB(EINSTART_S)
-  #include "mega/pins_EINSTART-S.h"             // ATmega1280, ATmega2560                 env:mega1280 env:mega2560
+  #include "mega/pins_EINSTART-S.h"             // ATmega1280, ATmega2560                 env:mega1280 env:mega2560ext
 #elif MB(WANHAO_ONEPLUS)
   #include "mega/pins_WANHAO_ONEPLUS.h"         // ATmega2560                             env:mega2560
 #elif MB(OVERLORD)

platformio.ini

 
 #
 # ATmega2560 with extended pins 70-85 defined
+# BOARD_BQ_ZUM_MEGA_3D
+# BOARD_ULTIMAIN_2
+# BOARD_MIGHTYBOARD_REVE
+# BOARD_EINSTART_S
 #
 [env:mega2560ext]
 platform            = atmelavr