(R26-3701.B) updater: Allow updating when one reboot does happened between two updates.

We used to rely on two reboots between two firmware updates. However for RW
firmware, after system reboots everything is already in RAM and never re-reads
from flash. That means we can eliminate one reboot if content of RW A firmware
is already identical to RW B firmware.

BUG=chromium-os:39282
TEST=emerge-daisy chromeos-firmware-daisy;
     Receive an update (or using fwb_tries=1, reboot) and then
     invoke chromeos-firmwareupdate --mode=autoupdate # success.

Original-Change-Id: Ie4b088ad94eca9061a8129f5242e2b69583d7221
Original-Change-Id: I3bc1f517b08f8a413dd7825eb6686d65ef0ddb5a
Original-Reviewed-on: https://gerrit.chromium.org/gerrit/44137
Original-Reviewed-on: https://gerrit.chromium.org/gerrit/44623

Change-Id: I63ef48ffa7f78c2a34419c646545dd337e6bde26
Reviewed-on: https://gerrit.chromium.org/gerrit/44613
Tested-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Commit-Queue: Hung-Te Lin <hungte@chromium.org>