Merge branch 'gh-7ec8f9a6/12/unknown/shell.nix' into 'master'

reform2-lpc-fw: add shell.nix

See merge request reform/reform!12

reform2-lpc-fw/shell.nix

+{ pkgs ? import <nixpkgs> { } }:
+pkgs.mkShell { buildInputs = with pkgs; [ stdenv gcc-arm-embedded ]; }