Map IDs for all groups referenced by Chromium OS udev rules.

Fixes #473 (need group "serial").
Create /media in the chroot if it does not exist.
Merge pull request #476 from drinkcat/add_groups

Map IDs for all groups referenced by Chromium OS udev rules.
Merge branch 'create_root_media' of https://github.com/drinkcat/chroagh