PlatformIO-related changes

- move platformio.ini out of source directory to be more consistent with 'normal' PlatformIO usage
  - facilitates IDE integration
- add related .gitignores

.gitignore

 #cmake
 CMakeLists.txt
 Marlin/CMakeLists.txt
+CMakeListsPrivate.txt
+
+#CLion
+cmake-build-*

Marlin/platformio.ini → platformio.ini

 # targets = upload
 
 [platformio]
-src_dir = ./
-envs_dir = ../.pioenvs
-lib_dir = ../.piolib
-libdeps_dir = ../.piolibdeps
+src_dir = Marlin
+envs_dir = .pioenvs
+lib_dir = .piolib
+libdeps_dir = .piolibdeps
 env_default = mega2560
 
 [common]