exclude metadata.xml from license check

The metadata.xml files should be excluded from the license checks. It's
just metadata about the package, and doesn't need a license string.
Moreover, the recommended license strings from the presubmit are
invalid XML that cause parsing errors.

BUG=None
TEST=pushed a change with metadata in board-overlays, hooks passed
RELEASE_NOTE=n/a

Change-Id: I230080e3e634594ad0e9b82a3deb524f50f36fd6
Reviewed-on: https://cos-review.googlesource.com/c/cos/repohooks/+/51831
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Vaibhav Rustagi <vaibhavrustagi@google.com>
diff --git a/pre-upload.py b/pre-upload.py
index b3944cf..d787ddd 100755
--- a/pre-upload.py
+++ b/pre-upload.py
@@ -112,6 +112,7 @@
     r'^(.*/)?README(\..*)?$',
     r'^(.*/)?LICENSE(\..*)?$',
     r'^(.*/)?toolbox$',
+    r'^(.*/)?metadata\.xml$',
 ]
 
 _CONFIG_FILE = 'PRESUBMIT.cfg'