updates.py: use update-system script if available

[antonlacon]

This changes the automatic check for updates to use /usr/bin/update-system if it's present. If not, then it falls back to asking the server for update information (current practice). update-system's output to include the checksum of the file intended to be downloaded. The addon doesn't do anything with it yet.

The other minor bits are some cleanups inside the function.

Needs https://github.com/LibreELEC/LibreELEC.tv/pull/7540 to see new behaviour.

Replaces #276.

[lrusak]

I would rather not call an external script. It should be integrated as a python import instead. Then if you want the cli script you can interface with that also.

[antonlacon]

This is based on #301, #303, and ~#307~. The relevant commits are 93d4ef1 and 747036a. Draft until other PRs merged.