Fix and optimize MightyBoard (#20493)

platformio.ini

@@ -457,7 +457,6 @@ board    = megaatmega2560
 [env:mega2560ext]
 platform            = atmelavr
 extends             = env:mega2560
-board               = megaatmega2560
 board_build.variant = megaextendedpins
 extra_scripts       = ${common.extra_scripts}
                       pre:buildroot/share/PlatformIO/scripts/copy_marlin_variant_to_framework.py
@@ -471,24 +470,31 @@ extends  = common_avr8
 board    = megaatmega1280
 
 #
-# MightyBoard ATmega2560 (MegaCore 100 pin boards variants)
+# MightyBoard AVR with extended pins
 #
-[env:MightyBoard1280]
-platform      = atmelavr
-extends       = common_avr8
-board         = ATmega1280
-upload_speed  = 57600
+[mega_extended_optimized]
+extends             = common_avr8
+board_build.variant = megaextendedpins
+extra_scripts       = ${common.extra_scripts}
+                      pre:buildroot/share/PlatformIO/scripts/copy_marlin_variant_to_framework.py
+upload_speed        = 57600
+build_flags         = ${common.build_flags} -fno-tree-scev-cprop -fno-split-wide-types -Wl,--relax -mcall-prologues
 
 #
-# MightyBoard ATmega2560 (MegaCore 100 pin boards variants)
+# MightyBoard ATmega1280
+#
+[env:MightyBoard1280]
+platform = atmelavr
+extends  = mega_extended_optimized
+board    = megamega1280
+
+#
+# MightyBoard ATmega2560
 #
 [env:MightyBoard2560]
-platform                  = atmelavr
-extends                   = common_avr8
-board                     = ATmega2560
-upload_protocol           = wiring
-upload_speed              = 57600
-board_upload.maximum_size = 253952
+platform = atmelavr
+extends  = mega_extended_optimized
+board    = megaatmega2560
 
 #
 # RAMBo