nixos: Make system.build a submodule with freeformType

This allows the values below it to be specified as options, while
remaining compatible with existing code.

nixos/modules/system/build.nix

@@ -6,12 +6,15 @@ in
   options = {
 
     system.build = mkOption {
-      internal = true;
       default = {};
-      type = with types; lazyAttrsOf (uniq unspecified);
       description = ''
         Attribute set of derivations used to set up the system.
       '';
+      type = types.submoduleWith {
+        modules = [{
+          freeformType = with types; lazyAttrsOf (uniq unspecified);
+        }];
+      };
     };
 
   };