micropython: use Nix make infra and build in parallel

  buildInputs = [ libffi readline ];

  buildPhase = ''

    runHook preBuild

    make -C mpy-cross

    make -C ports/unix

    runHook postBuild

  '';

  makeFlags = [ "-C" "ports/unix" ]; # also builds mpy-cross

  enableParallelBuilding = true;

  doCheck = true;