Licensing of the well-known runtimes

[muep]

Many consumers of flatpak runtimes will have (or at least should have) interest on how the content in them is licensed. Some GNU projects in particular want to choose runtimes that only contain free software.

If there is an intent to only ship free software in some runtime, it would be very useful to document this.

And then an extension that would take quite a bit more work... so then for users with more complex requirements, it would be useful to have a manifest with at least all the software names, versions and licenses for the entire content of a runtime. This now exists in partial form at least for the freedesktop runtime, but ideally this would be a bit more complete for the yocto-produced part and the format would be the same for the bits assembled at yocto-level and at flatpak-builder level.