Revamp the way we set up the bind mounts

volatile-binds: support for files as well as dirs

- 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.
- Add verified functional support for bind mounts of files as well, to handle things like resolv.conf.
- Enhance the implementation to handle dynamic operation based on whether / is read only, rather than always operating when the package is installed in the image.

Once this is merged, I can push a corresponding change to mel.conf to use this, and this resolves the issue where / wasn't ro, as well as multiple issues with the bind mounts not starting on boot, the systemd-random-seed failure, and makes /media writable, letting us resolve these three jira issues:

JIRA: SB-1504, SB-1351, SB-1386