brillo_update_payload: Add metadata_size_file flag

Add optional metadata_size_file flag to output metadata size to
the specified file.

BUG=b:24751934
TEST=brillo_update_payload with the new flag specified

Change-Id: Ic43dcadbb3624bdca824e55834b287151f00c75f
Reviewed-on: https://chromium-review.googlesource.com/304932
Commit-Ready: Jason Kusuma <jkusuma@chromium.org>
Tested-by: Jason Kusuma <jkusuma@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>
1 file changed