Repair dotfiles persistence feature by adding a symlink from /lib/live/mount/persistence to /live/persistence.

live-boot's "link" option generates symlinks to /lib/live/mount/persistence.

This is correct in the supported use case (persistence initialized at initramfs
time), because components/9990-main.sh does "Move all mountpoints below /live
into /root/lib/live/mount" after setting up persistence. Our own
config/chroot_local-includes/usr/local/sbin/live-persist, however, mounts stuff
in /live/persistence instead, so the generated links are wrong.

We have lots of Tails-specific stuff that depends on the /live/persistence path,
so I'd rather not change that, and instead I'm adding a compatibility symlinks.

Will-fix: #10784