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/system/boot/default.nix

@@ -0,0 +1,23 @@
+{ config, lib, ... }:
+let
+  cfg = config.my.system.boot;
+in
+{
+  options.my.system.boot = with lib; {
+    tmp = {
+      clean = mkEnableOption "clean `/tmp` on boot.";
+
+      tmpfs = my.mkDisableOption "mount `/tmp` as a tmpfs on boot.";
+    };
+  };
+
+  config = {
+    boot = {
+      tmp = {
+        cleanOnBoot = cfg.tmp.clean;
+
+        useTmpfs = cfg.tmp.tmpfs;
+      };
+    };
+  };
+}