lint/gitconfig: Enable checkpatch.pl checking of commits

This patch enables running the checkpatch script on the code portion
of each commit as part of the pre-commit hook. At this point there
is no checking of the commit message in place (e.g. for typos)

Change-Id: I7cdf0692cf372986e411f4aba4691417b73c7511
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: http://review.coreboot.org/8419
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marc.jones@se-eng.com>

util/gitconfig/pre-commit

@@ -1,2 +1,3 @@
 #!/bin/sh
-exec %MAKE% lint-stable
+%MAKE% lint-stable
+git diff --cached | util/lint/checkpatch.pl --no-signoff -q -