brillo_payload_generator: Adding warn function and flag verification

- Adding warn function
- Add check to verify that metadata_hash_file flag is set

BUG=b:25479363
TEST=brillo_payload_generator called successfully in local env

Change-Id: I1920c85c06be69cee93be74ef21d4f912714048d
Reviewed-on: https://chromium-review.googlesource.com/310889
Reviewed-by: Alex Deymo <deymo@chromium.org>
Tested-by: Jason Kusuma <jkusuma@chromium.org>
1 file changed