main/devicepkg-dev: add kernel scripts and config fragments

It simplifies kernel packaging and reduces some steps in the maintaining process. Kernel fork owners don't have to maintain kernel config in pmaports anymore as long as they have a defconfig in the fork

Using linux-postmarketos-grate as a test subject

Important note

This MR is to a kconfig-fragments and not to master, @craftyguy thinks this can be merged with just 1 approval instead of the usual 2. The 1 approval should probably come from either @jenneron or @craftyguy, ask us if you have questions.