Change in coreboot[master]: util/gitconfig/test: Add shellcheck test

[Martin Roth (Code Review)]

Martin Roth has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/23283 )

Change subject: util/gitconfig/test: Add shellcheck test
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Patch Set 6:

Hey Alex, if you don't object, I'd like to take over this patch.

Here are the things I want to do:
- move it to util/lint
- rename it to lint-24-shellcheck
- change it to accept a list of files and directories that are ready to be tested (similar to lint-000-license-headers and lint-stable-000-license-headers
- create a script lint-extended-24-shellcheck with that list of known good directories and files then add it to the extended lint-stable tests run on the builder.

I'd rather add it to lint-stable, but since people may not have shellcheck installed, that won't work.

Since shellcheck is still adding errors, we'll need to keep on top of that in new jenkins builders to make sure all the shell scripts still pass, similar to what needs to be done when we update GCC.