devserver: Install stateful with the same build of current rootfs.

This CL changes the devserver side, to add a new feature in cros-flash-based
provision framework: installing statefu partition with the same build of
current rootfs partition on an host if it has a super old build installed.

BUG=chromium:658374
TEST=Ran autoupdate_EndToEndTest first on host, then repair it with this new
feature. Proved that without this feature, repair fails. With this feature,
repair succeeds.

Change-Id: Ibcb9b5999d805282cc96b4bbe229e81d1dd0c173
Reviewed-on: https://chromium-review.googlesource.com/415545
Reviewed-by: Allen Li <ayatane@chromium.org>
Tested-by: Xixuan Wu <xixuan@chromium.org>
Commit-Queue: Xixuan Wu <xixuan@chromium.org>
2 files changed