Fix Mightyboard envs (#18194)

platformio.ini

@@ -94,7 +94,7 @@ board         = megaatmega1280
 [env:MightyBoard1280]
 platform      = atmelavr
 extends       = common_avr8
-board         = megaatmega1280
+board         = ATmega1280
 upload_speed  = 57600
 
 #
@@ -103,7 +103,7 @@ upload_speed  = 57600
 [env:MightyBoard2560]
 platform      = atmelavr
 extends       = common_avr8
-board         = megaatmega2560
+board         = ATmega2560
 upload_protocol = wiring
 upload_speed  = 57600
 board_upload.maximum_size = 253952