Provide downloadable packages for various use-cases

[KanjiMonster]

Currently any downloadable extra package is more of a side effect of Yocto recipes providing multiple packages when only one is requested to be installed.

To provide support for different use cases, we should actually provide a curated list of additional packages to install.

[KanjiMonster]

One possible solution would be to create packagegroups, which would be empty meta-packages with a list of runtime dependencies.

We could then define a packagegroup-bisdn-linux-extra, and build this as an additional target (or not, if you just want a quick local build).

[KanjiMonster]

Package groups would also enable us to define additional packagegroups for different usecases (we would need to define them though).