add termux device type with shared home-manager userspace module

modules/default.nix

@@ -9,22 +9,32 @@ let
       ...
     }:
     {
-      imports = with inputs; [
-        ./essentials
-        ./users.nix
-        ./options.nix
-        (./. + "/${deviceType}") # specific modules
+      # NixOS-only modules. termux runs nix-on-droid (its own module system,
+      # class = "nixOnDroid"): options like services.*, users.*, sops.*, disko.*
+      # and nixpkgs.overlays (flake assertion) do not exist there.
+      imports =
+        with inputs;
+        [
+          ./options.nix
+          (./. + "/${deviceType}") # specific modules
+        ]
+        ++ lib.optionals (deviceType != "termux") [
+          ./essentials
+          ./users.nix
 
-        home-manager.nixosModules.home-manager # home-manager module
-        # nix-index-database.nixosModules.nix-index # nix-index module
-        grub2-themes.nixosModules.default # grub2 themes module
-        sops-nix.nixosModules.sops # sops module
-        self.homeConfigurations.default.nixosModule # default homeConfigurations
-        disko.nixosModules.disko # disko module
-      ];
-      nixpkgs.overlays = with inputs; [
-        self.nixosOverlays.default
-      ];
+          home-manager.nixosModules.home-manager # home-manager module
+          # nix-index-database.nixosModules.nix-index # nix-index module
+          grub2-themes.nixosModules.default # grub2 themes module
+          sops-nix.nixosModules.sops # sops module
+          self.homeConfigurations.default.nixosModule # default homeConfigurations
+          disko.nixosModules.disko # disko module
+        ];
+      # nix-on-droid asserts that nixpkgs.* stays unset in flake mode.
+      nixpkgs.overlays = lib.mkIf (deviceType != "termux") (
+        with inputs; [
+          self.nixosOverlays.default
+        ]
+      );
       _module.args = {
         inputs = inputs;
         xlib = config.xlib;