dispatcher: uuu: Allow filesystem image as rootfs
Currently, it's only possible to apply overlay on disk images(e.g. wic) since ApplyOverlay requests (by default) a root_partition parameter.
Allow to use standard filesystem image (e.g ext4) by setting the ApplyOverlay() use_root_partition parameter to false when there is no root_parition defined, and so consider it as standard filesystem image.
Change-Id: If7ed27fbdcfa13cf3141db054107e737aed15922 Signed-off-by: Loic Poulain email@example.com