8260289: Unable to customize module lists after change JDK-8258411

Reviewed-by: ihse, alanb

make/common/Modules.gmk

@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2014, 2020, Oracle and/or its affiliates. All rights reserved.
+# Copyright (c) 2014, 2021, Oracle and/or its affiliates. All rights reserved.
 # DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 #
 # This code is free software; you can redistribute it and/or modify it
@@ -26,9 +26,6 @@
 ifndef _MODULES_GMK
 _MODULES_GMK := 1
 
-# Hook to include the corresponding custom file, if present.
-$(eval $(call IncludeCustomExtension, common/Modules.gmk))
-
 ################################################################################
 # Setup module sets for classloaders
 
@@ -48,6 +45,11 @@ include $(TOPDIR)/make/conf/docs-modules.conf
 
 include $(TOPDIR)/make/conf/build-module-sets.conf
 
+################################################################################
+# Hook to include the corresponding custom file, if present.
+# Allowing MODULE list extensions setup above.
+$(eval $(call IncludeCustomExtension, common/Modules.gmk))
+
 ################################################################################
 # Depending on the configuration, we might need to filter out some modules that
 # normally should have been included