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.
targets/audio: 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