Add non-free-firmware support.

As noted in the first commit:

Moving packages is still in progress though!

Direct link to latest update: https://lists.debian.org/debian-boot/2023/01/msg00235.html

I hope we'll manage to move all packages, which would make it much easier to document, and clearer to everyone.

(I'm purposefully leaving aside the complicated case of downloader packages found in contrib, which are free software, but relying on downloading stuff from a random vendor site/mirror, possibly accepting some EULA, etc. Those aren't useful in an installer context, and those happily stay in contrib where they belong.)