cros flash: restore stateful parition if it is corrupted
cros flash: restore stateful parition if it is corrupted

Cros Flash relies on certain packages installed on the stateful
partiiton such as Python to perform rootfs update. When the stateful
partition is corrupted, user can run --no-rootfs-update to try
restoring the stateful partition first. However, this is not known by
many users.

Cros Flash should check if the requirements for launching devserver
(necessary for rootfs update) are met, and ask user if they want to
restore the stateful partition first. The user prompt is necessary
because rootfs and stateful mismatch may lead to unexpcted results.

BUG=chromium:348675
TEST=Ran `cros flash` to flash a DUT with corrupted stateful partition

Change-Id: If234428549239ef09ba4235a24e35f307a66699e
Reviewed-on: https://chromium-review.googlesource.com/188887
Reviewed-by: Don Garrett <546ec366615570e68497e3c5df9092106a9808f4@chromium.org>
Commit-Queue: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>
Tested-by: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>

chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite