Add backward compatibility bind mount /lib/live/mount -> /run/live
The paths used in the current released versions of live-boot are a form of public API, and existing applications and scripts might rely on them. Mount the new path on the previous one so that they do not break on upgrade (see #886328).
@hertzog symlinks were not enough to un-break the software we have that depend on live-boot. I can make this optional if you like.