Handling already mounted persistence backing media correctly.

Handling already mounted persistence backing media correctly.

I.e. treat the case where a backing media already had been mounted in
the expected place like a success (by printing the mount dir). This
doesn't fix any bug in live-boot, but it will come in handy for the
(soon-to-come) script for activating custom persistence in user-space.

While we're at it, document this function.