futility: updater: Add --manifest to scan and print archive info

The firmware updater packages used to rely on a pre-generated VERSION
file to report what files were included and their image versions. Its
format was hard to parse, and may be out-dated if people repack without
updating VERSION file.

The firmware updater today has the ability to read and parse version,
key hash, ... etc everything we need, so it seems more reasonable to
just let firmware updater scan updater package and print the information
in JSON format, so it will be very easy to fetch latest information.

To make sure the output is purely JSON, the start and end messages are
now sent to stderr instead of stdout.

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

Change-Id: Ifa468fbb3adf798c7931f015258e6c6ce93de993
Signed-off-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1260804