nixos: create 'modules/nixos' folder

Let's consolidate all modules under one path, so that NixOS,
home-manager, and nix-darwin (if I ever end up using it down the line)
would go under the same folder.

modules/nixos/hardware/firmware/default.nix

@@ -0,0 +1,38 @@
+{ config, lib, ... }:
+let
+  cfg = config.my.hardware.firmware;
+in
+{
+  options.my.hardware.firmware = with lib; {
+    enable = my.mkDisableOption "firmware configuration";
+
+    cpuFlavor = mkOption {
+      type = with types; nullOr (enum [ "intel" "amd" ]);
+      default = null;
+      example = "intel";
+      description = "Which kind of CPU to activate micro-code updates";
+    };
+  };
+
+  config = lib.mkIf cfg.enable (lib.mkMerge [
+    {
+      hardware = {
+        enableRedistributableFirmware = true;
+      };
+    }
+
+    # Intel CPU
+    (lib.mkIf (cfg.cpuFlavor == "intel") {
+      hardware = {
+        cpu.intel.updateMicrocode = true;
+      };
+    })
+
+    # AMD CPU
+    (lib.mkIf (cfg.cpuFlavor == "amd") {
+      hardware = {
+        cpu.amd.updateMicrocode = true;
+      };
+    })
+  ]);
+}