Making the /live to /lib/live move for the persistence code.

Making the /live to /lib/live move for the persistence code.

Exposing /lib/live/mount/persistence in user-space is highly relevant
for the (soon-to-come) tool for activating persistence post-initramfs.
We want to use the same persistence code for both scenarios, and avoid
using different cases for them, so we make it only use /lib/live.

Similarly we also make sure that persistence modes using unions use
the same copy-on-write dir or "overlay" (i.e. /lib/live/mount/overlay)
in order to make them work as intended.