firmware: add shell.nix for CI/local repro

firmware/shell.nix

@@ -0,0 +1,23 @@
+# This is a nix-shell for use with the nix package manager.
+# If you have nix installed, you may simply run `nix-shell`
+# in this repo, and have all dependencies ready in the new shell.
+
+{pkgs ? import (builtins.fetchTarball {
+  # Descriptive name to make the store path easier to identify
+  name = "nixos-2021-05";
+  # Commit hash
+  url = "https://github.com/NixOS/nixpkgs/archive/refs/tags/21.05.tar.gz";
+  # Hash obtained using `nix-prefetch-url --unpack <url>`
+  sha256 = "1ckzhh24mgz6jd1xhfgx0i9mijk6xjqxwsshnvq789xsavrmsc36";
+}) {}}:
+
+
+pkgs.mkShell {
+  buildInputs = with pkgs;
+    [
+        yosys
+        nextpnr
+        icestorm
+    ];
+
+}