default.nix

let
  pkgs = import <nixpkgs> {
    crossSystem = "aarch64-linux";
    overlays = [
      (final: super: {
        makeModulesClosure = x:
        super.makeModulesClosure (x // { allowMissing = true; });
      })
    ];
  };
  configuration = { pkgs, modulesPath, ... }: {
    imports = [
      (modulesPath + "/installer/sd-card/sd-image-aarch64-installer.nix")
    ];

    boot.kernelPackages = pkgs.linuxPackages_rpi3;
  };
  nixos = pkgs.nixos configuration;
in nixos.config.system.build.sdImage

Makefile

all:
  nix-build -I nixpkgs=~/path/to/nixpkgs-checked-out-at-pr-156005