rootfs-common: refactor the common compression commands

This makes the compression extension available in a variable, so it
can be used by the fs-specific commands. In this patch, it is used
by iso9660. Following patches show more use cases.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>