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

nix/configuration/roles/nvme/default.nix

@@ -8,7 +8,18 @@
 {
   imports = [ ];
 
-  environment.systemPackages = with pkgs; [
-    nvme-cli
-  ];
+  options.me = {
+    nvme.enable = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+      example = true;
+      description = "Whether we want to install nvme.";
+    };
+  };
+
+  config = lib.mkIf config.me.nvme.enable {
+    environment.systemPackages = with pkgs; [
+      nvme-cli
+    ];
+  };
 }