installer: Allow host-bin restoration only.

Add a -b option to the installer that only restores host-bin
scripts, based on the target files installed in all existing
chroots.

Incidentally, this fixes #1106 as crouton -f tarball now only
restores host-bin (and not chroot-bin).
12-edit-chroot: Test host-bin restoration (crouton -b).
installer: Only expand variables if prepare script is needed