make-disk-image.nix: support additional filesystem contents

This makes make-disk-image.nix slightly more consistent with other image
builders we have. Unfortunately I duplicated some code in doing so, but
this is temporary duplication on the path to consolidating everything.
See https://github.com/NixOS/nixpkgs/issues/23052 for more details on that.

I'm also exposing the option in the amazon-image.nix maintainer module.