lint: create two classes of tests, stable and dev

lint: create two classes of tests, stable and dev

We have tests that pass (and should be enforced soonish) and those
that don't pass yet (and thus shouldn't break the build).

The plan is simple: As soon as a test passes, it's marked stable so
things remain that way.

"make lint" runs all tests,
"make lint-stable" runs only those that shouldn't fail.

Change-Id: Iaa85d71141606d9756e29b37c7a34c2a15e573ac
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/681
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>