dmsquash-live: add rd.live.join

[Biaogo]

Continue with question #687.

Support a union join on rootfs wiih another img.

This makes throw away layers possible, or starting different setups depending on rootfs and the join.img.

Example:

• the live bootloader features a base system and a desktop system entry, where the desktop fs resides in the join.img, while rootfs would be just a console base system.
• throw away live layer, the installer extracts the rootfs, while live session fs resides in the join.img, including the installer itself

Usage:

rd.live.join=$name.img