Add Mightyboard Mega env (#17861)

platformio.ini

 src_filter        = ${common.default_src_filter} +<src/HAL/AVR>
 
 #
+# MightyBoard ATmega2560 (MegaCore 100 pin boards variants)
+#
+[env:MightyBoard1280]
+platform          = atmelavr
+board             = ATmega1280
+board_build.f_cpu = 16000000L
+lib_deps          = ${common.lib_deps}
+  TMC26XStepper=https://github.com/trinamic/TMC26XStepper/archive/master.zip
+src_filter        = ${common.default_src_filter} +<src/HAL/AVR>
+
+#
+# MightyBoard ATmega2560 (MegaCore 100 pin boards variants)
+#
+[env:MightyBoard2560]
+platform          = atmelavr
+board             = ATmega2560
+board_build.f_cpu = 16000000L
+lib_deps          = ${common.lib_deps}
+  TMC26XStepper=https://github.com/trinamic/TMC26XStepper/archive/master.zip
+src_filter        = ${common.default_src_filter} +<src/HAL/AVR>
+
+#
 # RAMBo
 #
 [env:rambo]