Add option system.configurationRevision to record revision of top-level flake

nixos/modules/misc/version.nix

@@ -84,6 +84,12 @@ in
       description = "Default NixOS channel to which the root user is subscribed.";
     };
 
+    configurationRevision = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = "The Git revision of the top-level flake from which this configuration was built.";
+    };
+
   };
 
   config = {