fix compile issue under mac os

[hitsmaxft]

1. fix makefile compile error under darwin ; and move build target to root compile dir
2. add a nix shell.nix for compile and developing strtt

[phryniszak]

Thanks, looks good to me.
I have no way to verify mac build by I try to merge it anyway.

Could you please tell me what is the purpose of shell.nix with some references to nrf?

{ pkgs ? import <nixpkgs> {} }:

pkgs.mkShell {
  name = "nrf51-dev-environment";

  buildInputs = with pkgs; [
    inetutils
    openocd
    cmake
    ninja
    python3 # The nRF tools often require Python
    gcc-arm-embedded
  ];
  nativeBuildInputs = [
      pkgs.libusb1
  ];


  # Set the required environment variables
  shellHook = ''
  '';
}