Include a recipe to "seed" snaps

[om26er]

When someone is building a Yocto based system, they need to ship a few snaps by default as well.

To enable that, one needs a statically built snapd to run CLI commands, we also need to be able to define an environment variable that could be set in local.conf that our recipe would pick to "seed" snap packages by default.

[bboozzoo]

I guess this should start with having some documentation on the format of seed on classic systems, preferably hosted at docs.snapcraft.io