Project configuration required is too much

Looking at variables declared in freedesktop-sdk and shell customizations in gnome-build-meta, it looks like this is getting out of hand.

This is the way it is because as a rule, BuildStream should not be gaining knowledge specific to the payload it is building, and the burden is already much where we can see that it is undesirable to duplicate this when makeing gnome-build-meta depend on freedesktop-sdk through a junction element.

However, it would not make sense at all if one just wanted to build a simple a flatpak app on top of a flatpak SDK, and had to duplicate all this knowledge which is already in the SDK BuildStream project.