nixos/etc: create uninitialized /etc/machine-id with readonly /etc/ (#523894)

nixos/modules/system/etc/etc-activation.nix

@@ -173,5 +173,18 @@
 
     })
 
+    (lib.mkIf (config.system.etc.overlay.enable && !config.system.etc.overlay.mutable) {
+      # Systemd requires /etc/machine-id exists or can be initialized on first
+      # boot. This file should not be part of an image or system config because
+      # it is unique to the machine, so it is initialized at first boot and
+      # persisted in the system state directory, /var/lib/nixos.
+      environment.etc."machine-id".source = lib.mkDefault "/var/lib/nixos/machine-id";
+      boot.initrd.systemd.tmpfiles.settings.machine-id."/sysroot/var/lib/nixos/machine-id".f =
+        lib.mkDefault
+          {
+            argument = "uninitialized";
+          };
+    })
+
   ];
 }