futility: cmd_update: Implement updater logic "FULL UPDATE".

The logic is same as --mode=factory or --mode=recovery,--wp=0 in legacy
firmware updater.

BUG=chromium:875551
TEST=make futil; futility update -i IMAGE
     tests/futility/run_test_scripts.sh $(pwd)/build/futility
BRANCH=None

Change-Id: Ifbfc4fb76f954483e779c8b508377d07561b67da
Signed-off-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1183651
Reviewed-by: Randall Spangler <rspangler@chromium.org>
2 files changed