Patch platformio.ini to fix build error in DevIoT

For compatibility with DevIoT, until that can be solved.

.gitignore

 .pioenvs
 .piolib
 .piolibdeps
+lib/readme.txt
 
 #Visual Studio
 *.sln

Marlin/platformio.ini → platformio.ini

 [env:mega2560]
 platform = atmelavr
 framework = arduino
-board = megaatmega2560
+board = mega2560
 build_flags = -I $BUILDSRC_DIR
 board_f_cpu = 16000000L
 lib_deps = ${common.lib_deps}
 [env:mega1280]
 platform = atmelavr
 framework = arduino
-board = megaatmega1280
+board = mega1280
 build_flags = -I $BUILDSRC_DIR
 board_f_cpu = 16000000L
 lib_deps = ${common.lib_deps}