Detect and disallow firmware downgrades

[Lanchon]

Right now, the latest available downloadable firmware is assumed to never be older than the currently installed firmware. The currently installed firmware version is never queried. A per-device automatically maintained local repository of downloaded firmware images and configuration backups is leveraged to ensure that upgrades are applied only once per device.

TODO: Query actual installed firmware versions and disallow downgrades.

[slycordinator]

The current, installed revision can be extracted from /tmp/loginprompt
It's the only file I could find that contains it.

Here's an excerpt of what I used in my own update script
temp_dir='/tmp'
login_prompt="$temp_dir/loginprompt"
...
installed_rev="$(sed -n '1p' "$login_prompt"|awk '{print $2}'|sed 's/^.*r/r/')"