crouton in a separate partition

The times of disastrous wipes and powerwashes are over!

- Create a separate partition for crouton (`/var/crouton`). The scripts stay in `/usr/local/bin`, but the chroots are stored in `/var/crouton/chroots`.
- Autodetect if the partition exists in `enter-chroot` and other scripts. If it does, mount it, and use it.
- Allow to restore scripts with the new `-b` option in the installer.

This is based on #630 (first 3 commits here).

A draft of the companion wiki page is available here: https://github.com/drinkcat/chroagh/wiki/chroots-in-a-separate-partition , I wrote it in the way that potential testers can try it out as it is.