util/lint: Introduce linter for git sign-off-by line

For compliance and to check if the commit message matches the
development guidelines, add a linter checking for the sign-off-by line
in the commit message. Also, hook it up to the test_build.sh script
which is used for CI builds.

These scripts were copied from the coreboot repository and were adjusted
so that they work for flashrom.

Change-Id: I03f1827803f8492d60a0a44174d5822c2265bfcb
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/70079
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>

util/lint/lint-extended-020-signed-off-by

@@ -0,0 +1,23 @@
+#!/usr/bin/env sh
+# SPDX-License-Identifier: GPL-2.0-or-later
+#
+# DESCR: Check for a signed-off-by line on the latest commit
+
+
+LINTDIR="$(
+  cd -- "$(dirname "$0")" > /dev/null 2>&1 || return
+  pwd -P
+)"
+
+# shellcheck source=helper_functions.sh
+. "${LINTDIR}/helper_functions.sh"
+
+if [ "${IN_GIT_TREE}" -eq 0 ]; then
+	exit 0
+fi
+
+# This test is mainly for the jenkins server
+if ! ${GIT} log -n 1 | grep -q '[[:space:]]\+Signed-off-by: '; then
+	echo "No Signed-off-by line in commit message"
+	exit 1
+fi