modules: Add shared-rootfs

Used to undo some of the close-knit integration work NixOS does, so we
can cheat an make a more generic rootfs.

modules/module-list.nix

@@ -35,6 +35,7 @@
   ./nixpkgs.nix
   ./quirks
   ./rootfs.nix
+  ./shared-rootfs.nix
   ./system-target.nix
   ./system-types.nix
 ]

modules/shared-rootfs.nix

@@ -0,0 +1,35 @@
+{ config, lib, ... }:
+
+let
+  inherit (config.mobile.rootfs.shared) enabled;
+  inherit (lib) mkIf mkOption types;
+in
+{
+  options = {
+    mobile.rootfs.shared.enabled = mkOption {
+      internal = true;
+      type = types.bool;
+      default = false;
+      description = ''
+        Enable when building a generic rootfs that does not include a kernel image.
+
+        This makes sturdier rootfs that work on different devices.
+      '';
+    };
+  };
+
+  config = mkIf enabled {
+    # Ensure we don't bring a kernel or initrd into the system.
+    system.boot.loader.kernelFile = "dummy";
+    system.boot.loader.initrdFile = "dummy";
+
+    # And totally obliterate device-specific files from stage-2.
+    system.extraSystemBuilderCmds = ''
+      echo ":: Removing non-generic system items..."
+      (
+        cd $out
+        rm -vf dtbs initrd kernel kernel-modules kernel-params
+      )
+    '';
+  };
+}