Add private, build and vscode folder to .gitignore

The private folder is for convenience, for putting some files away and
for own TODO lists; generally for data which is related to the project
but shouldn't be committed.

The build folder contains intermediate/object files for building the
different stages; it should be ignored explicitly, even though the
file ending of all files marks the folder as ignored implicitly.

.gitignore

@@ -7,3 +7,8 @@
 *.hex
 *.o
 *.so
+
+# Full ignored folders
+*.build/
+*.private/
+*.vscode/