build_kernel_image: fix duplicate options

Don't append common options twice.

BUG=chromium-os:33220
TEST=build image, check bootargs

Change-Id: I632b6a9c7726ff3838e84c51ed8cea77142ba948
Reviewed-on: https://gerrit.chromium.org/gerrit/29134
Reviewed-by: Gaurav Shah <gauravsh@chromium.org>
Reviewed-by: Puneet Kumar <puneetster@chromium.org>
Tested-by: Puneet Kumar <puneetster@chromium.org>
Commit-Ready: Puneet Kumar <puneetster@chromium.org>
1 file changed