Include meta-flatpak and make it part of the m-g-d

This story is about adding the meta-flatpak repository to g-d-p and making sure the recipes are available.

AC:

• The meta-flatpak is added as a submodule in the g-d-p repository.
• The meta-flatpak repository is included in the g-d-p configurations.
• You are able to build bubblewrap (bwrap) manually.