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/helper_functions.sh

@@ -33,7 +33,8 @@ fi
 if [ "${IN_GIT_TREE}" -eq 1 ]; then
 	FIND_FILES="${GIT} ls-files"
 else
-	FIND_FILES="find src"
+	FIND_FILES="find "
+	FINDOPTS="-type f"
 fi
 
 # Use git grep if the code is in a git repo, otherwise use grep.