Motivation for this change

Add expression for building the kernel module 'efi-runtime'
to enable FWTS testing of EFI bits.

Builds from FWTS source, but separately so it matches a given kernel.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • 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 nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Once installed and loaded,
with modprobe efi_runtime or insmod the module file directly,
this enables UEFI tests like so:

$ sudo fwts --uefitests

Without the module, most of the tests do nothing (abort as unsupported).