Move platformio directories out of source tree

.gitignore

 # Debug files
 *.dSYM/
 *.su
+
+#PlatformIO files/dirs
+.pioenvs
+.piolib

Marlin/platformio.ini

 
 [platformio]
 src_dir = ./
+envs_dir = ../.pioenvs
+lib_dir = ../.piolib
 env_default = mega2560
 
 [env:mega2560]