reform2-lpc-fw: add shell.nix

Nix users can enter a shell with the appropriate toolchain by issuing the
`nix-shell` command in the reform2-lpc-fw directory.

reform2-lpc-fw/shell.nix

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