Adding support for read/write filesystem images

A user can provide a filesystem image (rootfs.img) inside a compressed
tarball and that filesystem image will be mounted read/write.  This provides
some benefits over a device mapper snapshot overlay, especially when the
live system becomes full.  The boot command line simple needs
"rd.writable.fsimg" added to utilize this feature.

Additional documentation for this option as well as other live boot
options is included.

Signed-off-by: Major Hayden <major@mhtx.net>