Restructure flake.nix for a simpler config for building different images off the same NixOS config.

nix/configuration/roles/iso/default.nix

@@ -1,18 +1,22 @@
 {
-  config,
   lib,
-  pkgs,
   ...
 }:
 
 {
   imports = [ ];
 
-  options.me.buildingIso = lib.mkOption {
+  options.me.buildingPortable = lib.mkOption {
     type = lib.types.bool;
     default = false;
     example = true;
-    description = "Whether we are building an ISO image.";
+    description = "Whether we are building a portable image (iso/sd). This would disable CPU-specific optimizations and persistent file mounts.";
   };
 
+  options.me.mountPersistence = lib.mkOption {
+    type = lib.types.bool;
+    default = false;
+    example = true;
+    description = "Whether we should mount persistent directories.";
+  };
 }