add nix-shell file for firmware compilation

shell-firmware.nix

@@ -0,0 +1,8 @@
+{ pkgs ? import <nixpkgs> {}}:
+let
+  hx = import ./derivations.nix { inherit pkgs; };
+in
+  pkgs.mkShell {
+  	buildInputs = [ hx.rustc hx.rust-riscv32imc-crates hx.cargo ];
+  	RUSTFLAGS="-L ${hx.rust-riscv32imc-crates}/lib/rustlib/riscv32imc-unknown-none-elf/lib";
+  }