postmarketos-initramfs: better wait_[br]oot_partition

Rework the messages and avoid re-spawning the splash on every check.
Additionally, add a call to check_keys, this lets users drop to a debug
shell or export logs to triage this failure (particularly useful for
hard-to-reproduce issues).

Finally, add a (liberal) 30 second timeout and dump logs at the end. If
we're just waiting for a device to show up and it doesn't after 30
seconds then it's probably never going to...

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>