enter-chroot: mount a local dir as the home dir, if present.

Situation: I don't like to encrypt the whole chroot, but I'd like my home directory (in chroot) to be encrypted on disk. The solution suggested [here](http://digitalconsumption.com/forum/721-Crouton-Encrypted-User-Folders) is to 'piggyback' on the ecryptfs present in Chrome OS, pretty much the way `Downloads` directory works.

So, the patch changes the `enter-chroot` script, and what is does is that it checks whether there is a directory named `localuserhome-${chroot-name}` in `/home/chronos/user`, and it there is, it bind-mounts it as user's home directory in the chroot.

I've been using it without problems for a while, but there might be some collisions in the grand scheme of things that I'm not aware of.