dmsquash-live: Support a flattened squashfs.img

dmsquash-live: Support a flattened squashfs.img

A simplified root filesystem structure may be provided for OverlayFS
overlays by squashing the root filesystem directly instead of squashing
an embedded image file at /LiveOS/rootfs.img.  Detect and configure
such a squashed root filesystem for live booting.

For OverlayFS boots, avoid the read-only Device-mapper linear device
  at /dev/mapper/live-base.
Create a consistent device link at /dev/live-base for the read-only
  base loop device for all overlayed live root filesystems.
Consistently provide a link at /dev/root for wait_for_dev.

Update documentation.