Add lint_diff.sh, which runs checkpatch on git diff to either HEAD~1 (if
the tree is clean) or HEAD. This can be used as pre-commit hook, and
it's what jenkins will run.
Add lint_all.sh, which runs checkpatch on a whole repository to test if
the rules we are checking for make sense in Osmocom context.
Related: OS#5087
Change-Id: I1d02c169b05fb05b87209a444a5ddb86efc99d04
