[autotest] Do not return default CrOS stable version for firmware stable version

This CL fixed an issue in CrOSHost.get_repair_image_name method. For requests to
get stable firmware version, it should not return the default stable version for
CrOS image. The issue does not break anything, but it causes repair to waste
time on staging a none-existing build.

BUG=chromium:555008
TEST=local
import common
from autotest_lib.server import hosts
h = hosts.create_host('chromeos2-row3-rack7-host1')
h.get_repair_image_name() # Return the default stable version for CrOS
h.get_repair_image_name(image_type='firmware') # Return None

Change-Id: I13ba55607179529b68dcf2e8ae0357f2abbc2007
Reviewed-on: https://chromium-review.googlesource.com/312507
Commit-Ready: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
Reviewed-by: danny chan <dchan@chromium.org>
Reviewed-by: Fang Deng <fdeng@chromium.org>
1 file changed