Rework manual update for new infrastructure

[antonlacon]

This updates the addon to allow manual updates to work with nightly images off test.libreelec.tv.

The first five commits are cleanups, mostly around logging and if tests.

The sixth commit is an enhancement to add "https://" to a Custom URL if the string doesn't start with http:// or https://. Ex: I can put test.libreelec.tv as the Custom URL, as it'll correct it to https://test.libreelec.tv/releases.json.

The seventh commit updates the list of available builds in the releases.json displayed to users to account for it having images in addition to tarballs.

The eighth commit changes what get_available_builds() returns when given an argument from the build's name to the build's subpath+name. This is only used when combining with the URL from the releases.json in do_manual_update().

The ninth commit is documentation.

Caveats: get_available_builds() has a matches variable. I don't know what situation takes this path / LE provided releases.json don't trigger this path. It was put in recently, so I've kept it and only done enough to keep it from causing the addon to error from not having a 'file' entry. Matches is the result of the releases.json's prettyname_regex being applied to the filename.

Uboot devices allow selecting any of the images from their project. Ex: an A64.arm device will show 40 images (4 devices * 10 images each), any of which may be chosen.

Tested RPi2 & A64 by spoofing and seeing what it downloads.

[flubshi]

Tested on raspberry pi4: For manual update (channel: LibreELEC-11.0 @ test.libreelec.tv) the list of avilable versions seems to contain entries, but with an empty label.

[antonlacon]

I believes labels are from the prettyname_regex in the releases.json. It's set up for release images, and I'm not sure what a prettyname for a nightly would reduce it to. I can see taking the distroname off the front if length is an issue. What were you expecting for a label on them?

[flubshi]

For the label I would expect the pretty name and, if not matching, the filename as fallback. I think that's the way it was before.

[flubshi]

Works on raspberry pi4 👍

[antonlacon]

The pretty name in the releases.json is set up for release builds. Nightlies are named differently, so they fallback to the filename. The addon already stripped '.tar' off filenames; I extended it to take '.img.gz' off too. I've also added a commit to remove the distro + project.arch off the front.

Before: LibreELEC-RPi2.arm-11.0-nightly-20220901-a1a77bf.img.gz
After: 11.0-nightly-20220901-a1a77bf

I believe that's equivalent enough to what the prettyname regex was creating: the version of the file.