This give users the choice to have rootfs formatted with squashfs.
When --squash_image is specified, the rootfs will be formatted to squashfs.
Users can also use --squash_sort_file to specify the file priority when
squashfs is created.
BUG=None
TEST=Manually tested "--squash_image", and the image can be installed
from USB stick. Also tried "--squash_sort_file=sort-prio.list", and files
in squashfs are sorted.
Change-Id: I5fd818ac9d1203598926efa82e94fa105cd86ebc
Reviewed-on: http://gerrit.chromium.org/gerrit/5664
Tested-by: Da Zheng <zhengda@chromium.org>
Reviewed-by: Da Zheng <zhengda@chromium.org>
