Fix bug where cros_factory_install didn't end up in the kernel cmdline

BUG=none
TEST=./build_image --board=stumpy factory_install, verify kernel cmdline

Change-Id: Icdf3156ea08e46d0ce5154133d62cb8d98d144f5
Reviewed-on: https://gerrit.chromium.org/gerrit/34102
Tested-by: Liam McLoughlin <lmcloughlin@chromium.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Commit-Ready: Liam McLoughlin <lmcloughlin@chromium.org>

build_library/build_image_util.sh

@@ -129,6 +129,7 @@ create_boot_desc() {
   --arch="${ARCH}"
   --keys_dir="${DEVKEYSDIR}"
   --usb_disk="${FLAGS_usb_disk}"
+  --boot_args="${FLAGS_boot_args}"
   --nocleanup_dirs
   --verity_algorithm=sha1
   ${enable_rootfs_verification_flag}