(this doesn't work)

mipskernel.nix

with import <nixpkgs> {
  crossSystem = {
    system = "mipsel-linux-gnu";
    withTLS = true;
    platform = {
      uboot = null;
      name = "malta";
      kernelArch = "mips";
      kernelBaseConfig = "malta_defconfig";
      kernelHeadersBaseConfig = "malta_defconfig";
      kernelTarget = "uImage";
      gcc = { abi = "gnu"; } ;
      kernelAutoModules = false;
      kernelModules = false;      
      kernelPreferBuiltin = true;
    };
  };
};

let nocheck = d : lib.overrideDerivation d (a: {
  doCheck = stdenv.buildPlatform == stdenv.hostPlatform;
  }); 
in rec {
  hello = (nocheck pkgs.hello);
  linux = linux_latest;
  kernel = lib.overrideDerivation linux (a: {
    name = "nixwrt_kernel";
  });
  all = buildEnv {
    name = "nixwrt";
    paths = [ hello pkgs.busybox ];
  };
}

For the avoidance of doubt, the command I'm running is

NIX_PATH=nixpkgs=`pwd`/nixpkgs nix-build -A linux --show-trace