Motivation for this change

I prefer to run a deblobbed kernel for various reasons and I know I'm not the only one. This is fairly easy to do with linuxPackages_custom by just dropping in a custom tarball. But since this is so simple I figured it would be nice to add support so others interested in running a deblobbed kernel don't have to mess around with custom linux packages.

This is pretty much a rough draft as I am almost certain there is a better way to accomplish this. Any feedback or recommendations are welcome.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • [x NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.