util/lint: Fix linting outside of git repos

If the coreboot code is not in a git repository, the linters switch
from using `git ls-files` to find.  This requires some changes to
prevent the linters from looking at the wrong files which are
automatically excluded by git.

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: I81d138760c29a7c476280bb9d963f6be99c75d6d
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68475
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Fred Reitberger <reitbergerfred@gmail.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

util/lint/lint-stable-009-old-licenses

@@ -28,7 +28,11 @@ HEADER_EXCLUDED="\
 _shipped$|\
 ^util/scripts/no-fsf-addresses.sh|\
 ^util/lint/lint-000-license-headers|\
-^util/lint/lint-stable-009-old-licenses\
+^util/lint/lint-stable-009-old-licenses|\
+^util/nvidia/cbootimage|\
+^3rdparty|\
+__pycache__|\
+^payloads/external\
 "
 
 if [ -z "$HEADER_DIRS" ]; then