Store the default config in chroot-etc, install it using CHROOTETC, and create the link in enter-chroot after the user is known, and if that file exists in /etc/crouton.

The touch target really should be using CHROOTETC as well, but I never got around to updating it.  I think it would make sense to eventually add some generic code to enter-chroot to support a home folder structure in /etc/crouton/home whose files get symlinked in the relevant locations on login, but don't do this for this PR; just add the one-off to enter-chroot for now.