Symlink to backup dir until ephemeral storage available

Currently, when you call `register-state-path --leave-symlink /a`, a
symlink is left at /a pointing to /mnt/state/a. However, /mnt/state
doesn't point at ephemeral storage until the instance has booted. So if
another element tries to write through the symlink, it goes wrong.

This patch defers moving content (and leaving a symlink if required) to the
backup dir until finalise.d runs.

Change-Id: Id5bc25965db1fdf27f2bcb947c518f6f4566fd2f
Co-Author: Nicholas Randon
Merge "Symlink to backup dir until ephemeral storage available"