build_image: Display progress bar message on stderr

Currently, progress bar messages are displayed on stdout through
echo. Move these messages to stderr so they are in the same
stream as the logging messages.

BUG=none
TEST=manual tests

Change-Id: Id20db4ecef51ddb1eb07ab13e83f8f43803c6410
Reviewed-on: https://chromium-review.googlesource.com/275726
Reviewed-by: David Pursell <dpursell@chromium.org>
Trybot-Ready: Ralph Nathan <ralphnathan@chromium.org>
Commit-Queue: Ralph Nathan <ralphnathan@chromium.org>
Tested-by: Ralph Nathan <ralphnathan@chromium.org>
1 file changed