Revamp the way we set up the bind mounts

- Drop some legacy remnants
- Drop mount-binds and ro-binds in favor of volatile-binds, which is
  implemented quite differently. Rather than having a single service file
  which tries to do everything, and which only gets installed for read only
  images, now we generate per-mount service files with appropriate mount based
  dependencies, which integrate much better, and which are configured with
  a conditional such that they only run based on the rootfs being read only.
- Fix the failure to start systemd-random-seed.service.
- Ensure that /media is also writable, for udisks automount of media.
- Update readme to match the current state of the layer.

JIRA: SB-1504, SB-1351

Signed-off-by: Christopher Larson <kergoth@gmail.com>