lint: update signed-off-by check

The current signed-off-by check doesn't really work well as a lint
stable test being run by the pre-commit git hook.  This test looks
at the PREVIOUS commit instead of the staged commit, which works well
on the server. In looking for a way to check the staged commit message,
I found the commit-msg hook, which is the correct way to check the
staged message.

- Update the commit message check from a lint-stable test to a
lint-extended test.
- Add the check for signed-off-by to the commit-msg hook

Change-Id: I8be8aef25730d988c7cf1586ae66ecb839d5f756
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20920
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Chris Ching <chingcodes@chromium.org>

util/gitconfig/commit-msg

@@ -169,5 +169,8 @@ _gen_ChangeId() {
 	git hash-object -t commit --stdin
 }
 
-
+if ! grep -qi '^[[:space:]]*\+Signed-off-by:' "$MSG"; then
+	printf "\nError: No Signed-off-by line in the commit message.\n"
+	exit 1
+fi
 add_ChangeId