Got the firmware to compile and produce something vaguely sensible.

firmware/shell.nix

@@ -0,0 +1,10 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell {
+  buildInputs = [
+    pkgs.stdenv
+
+    # keep this line if you use bash
+    pkgs.bashInteractive
+  ];
+}