Dm: init: Enable rootfs mount as dm-verity during boot without ramdisk

If the rootfs image has HASH tree appended, we need mount the
rootfs as dm-verity in boot phase. This commit make rootfs mount
as dm-verity without ramdisk.

Change-Id: I10329ba09d57b832482cd9b5f668acc88bddf548
Signed-off-by: Shumin Qiu <shuminq@codeaurora.org>