RFC: Audio auto update

Work in progress, this surely breaks stuff (like custom targets). @dnschneid I'd like your input before I move forward.

So I'm trying to find ways to auto-update the CRAS plugin.

One way we discussed, a while ago, was to download the latest version of crouton, and update with `-U audio`. I'm not too comfortable with that as it will break chroagh (or other fork/branch), and it requires that the checking code is placed in `enter-chroot`, which, really, shouldn't be the place for that kind of checks.

The approach here is to output the generated target files to the chroot into separate files in `/usr/local/crouton/`, and source them from `prepare.sh`. Then, we can update the audio target from inside the chroot by sourcing these files (see `cras-audio-update`).
Finally `enter-chroot` will just need to call `cras-audio-update` just after `/etc/rc.local` (or another better way, to be defined).

We'll probably also want to split `core`/`post-common` to contain only what is really needed by `audio`.
Wouldn't /etc/crouton be a better place, primarily to keep everything together?