pre-upload: split checkpatch check into a hook

BUG=chromium:466264
TEST=`./pre-upload_unittest.py` passes

Change-Id: I2a818745e35d49ed00a165724c1f6cf8acc83b95
Reviewed-on: https://chromium-review.googlesource.com/262395
Reviewed-by: Steve Fung <stevefung@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/pre-upload.py b/pre-upload.py
index a1a8554..383f33e 100755
--- a/pre-upload.py
+++ b/pre-upload.py
@@ -1147,6 +1147,7 @@
 # A dictionary of flags (keys) that can appear in the config file, and the hook
 # that the flag controls (value).
 _HOOK_FLAGS = {
+    'checkpatch_check': _run_checkpatch,
     'stray_whitespace_check': _check_no_stray_whitespace,
     'json_check': _run_json_check,
     'long_line_check': _check_no_long_lines,