Track deployment root/inode from prepare root

Track deployment root/inode from prepare root

When we added composefs, it broke the logic for detecting the booted
deployment which was previously a direct (device, inode) comparison.
So the code there started looking at `etc`.  However, that in
turns breaks with `etc.transient = true` enabled.

Fix all of this by tracking the real deployment directory's
(device,inode) that we found in `ostree-prepare-root`, and inject
it into the extensible metadata we have in `/run/ostree-booted`
which is designed exactly to pass state between the initramfs
and the real root.

Signed-off-by: Colin Walters <walters@verbum.org>