tests/30-audio: Fix dd command, better pulseaudio test.
installer: Generate target files in /etc/crouton/installer and source them

This keeps a copy of the target file that we can use for automatic
audio update later on.
chroot-bin/cras-audio-update: Facilities for CRAS plugin auto-update
targets/common: Do not print targets with blank descriptions
core: Move active code out of installer/*/prepare, split core.

core.minimal contains only the bare minimum for other targets
to run.
post-common: Split in necessary and minimal files
enter-chroot: Run cras-audio-update.

I wanted to integrate it better with rc.local, but rc.local runs
with all FDs closed (for good reasons), while we want user
interactivity.
tests/30-audio: Very basic test of autoupdate