build_image: Only enable serial output on test image.

Modified from https://chromium-review.googlesource.com/#/c/263031/
Since these arguements will break the signer test in the buildbot, we
only enable them for the test image.

BUG=chrome-os-partner:44546
    chrome-os-partner:44601
TEST=Check the kernel log is printed
     Wifi driver can be reloaded

Change-Id: Ic233a6462922ce45f766d1918ed755f798375acb
Reviewed-on: https://chromium-review.googlesource.com/296915
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Commit-Queue: Chih-Yu Huang <akahuang@chromium.org>
Tested-by: Chih-Yu Huang <akahuang@chromium.org>
1 file changed