use ~/Downloads as home directory for chroot user

This patch mounts your ~/Downloads directory as the home directory for
the main user in all of your Crouton chroots.  By doing this, you get:

* easy access to files in/beneath your home directory from Chromium OS
* an encrypted home directory because ~/Downloads is already encrypted
* the same, persistent home directory across different Crouton chroots
store chroots in /home/chronos for free encryption
Merge branch 'bindmount-Downloads-as-HOME'