Add message for Hammer firmware updater use.

This CL adds the boot message for hammer firmware update. When the
hammer needs update at boot time, then we show the message before
entering UI.

BUG=b:65180379
TEST=With new firmware, plug hammer and reboot poppy,
     check the boot message shows up.
CQ-DEPEND=CL:651788

Change-Id: Ie0bf530571e39c0514a1bd5721e0ead8217a0805
Reviewed-on: https://chromium-review.googlesource.com/656859
Commit-Ready: Chih-Yu Huang <akahuang@chromium.org>
Tested-by: Chih-Yu Huang <akahuang@chromium.org>
Reviewed-by: Joel Kitching <kitching@chromium.org>
Reviewed-by: Wei-Ning Huang <wnhuang@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
3 files changed