Create a symbolic link from /usr/local/chroots to /mnt/stateful_partition/crouton:
Makes sure that dev_install does not wipe the chroots.

Also, migrate existing chroots to new directory (transparent to the user, and very
fast as we stay on the same filesystem).
Fixes after review.
Install chroots in /mnt/stateful_partition/crouton/chroots
so we can put other stuff in /mnt/stateful_partition/crouton (e.g. shared directories).