pre-upload: Do not warn about copyright lines in checkpatch.pl

The checkpatch.pl script comes from the Linux kernel tree and follows
those copyright rules.  Add an --exclude_regex to cros_license_check so
that we do not warn about it when updating it in repohooks (while
preserving the check for the rest of this repo.)

BUG=None
TEST=Wrote a test commit on checkpatch.pl and tried to repo upload it.

Change-Id: Ia51f11d814a333cadd730864d09bec542225c252
Signed-off-by: Filipe Brandenburger <filbranden@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/304867
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/PRESUBMIT.cfg b/PRESUBMIT.cfg
index af59bca..b1b1d78 100644
--- a/PRESUBMIT.cfg
+++ b/PRESUBMIT.cfg
@@ -1,3 +1,9 @@
 [Hook Scripts]
 hook0 = ../../chromite/bin/cros lint ${PRESUBMIT_FILES}
 hook1 = ./pre-upload_unittest.py
+
+[Hook Overrides]
+cros_license_check: true
+
+[Hook Overrides Options]
+cros_license_check: --exclude_regex=\bcheckpatch\.pl$