Move the boot args for test image to test_image_util.sh

When the buildbot builds the base image and the test image, the
arguments of the test image would be populated to the base image.
So move the setting from build_image to test_image_util.sh.

BUG=chrome-os-partner:45026
TEST=run './build_image base test' and check the kernel args of two
     image is as expected.

Change-Id: Icabf95c21ed5015e2e979b25a154431e7c9aa91a
Reviewed-on: https://chromium-review.googlesource.com/297684
Reviewed-by: Chih-Yu Huang <akahuang@chromium.org>
Commit-Queue: Chih-Yu Huang <akahuang@chromium.org>
Tested-by: Chih-Yu Huang <akahuang@chromium.org>
2 files changed