build_image: Add messages to make parsing the output easier.

Currently, it is very hard to automatically parse the output to create a
progress bar. Add extra messages that will be displayed when a progress
bar is being used.

BUG=brillo:856
TEST=manual test

Change-Id: I03c6058d26bca9c003adbc1087962365c421f2d8
Reviewed-on: https://chromium-review.googlesource.com/266722
Reviewed-by: Steve Fung <stevefung@chromium.org>
Tested-by: Ralph Nathan <ralphnathan@chromium.org>
Commit-Queue: Ralph Nathan <ralphnathan@chromium.org>
Trybot-Ready: Ralph Nathan <ralphnathan@chromium.org>
1 file changed