stateful_update: support retrieving payloads locally

Currently, stateful_update either accepts a URL to download the
payload (stateful.tgz) or reads the devserver URL from
/etc/lsb-release. This CL allows stateful_update to accept a local
payload path. This enable reading the payload directly from stdin with
a '-' path, so that we can pipe the payload through an SSH connection.

BUG=chromium:327588
TEST=stateful updates on a local peppy

Change-Id: Ife7b8c4ce0b667ccbbdbad9f2225ee23c1a8111a
Reviewed-on: https://chromium-review.googlesource.com/179533
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Yu-Ju Hong <yjhong@chromium.org>
Tested-by: Yu-Ju Hong <yjhong@chromium.org>
1 file changed