RFR: JDK-8259942: Enable customizations in CompileJavaModules.gmk and Main.gmk

[Adam Farley]

Hi All,

Sponsor requested for a simple fix (reviews already complete).

This change restores the ability of these makefiles to include
customizations.

No sign this ability was removed deliberately, so I think it was just a
mistake.

Also includes a small readability improvement.

Bug: https://bugs.openjdk.java.net/browse/JDK-8259942
PR: https://github.com/openjdk/jdk/pull/2134

Best Regards

Adam Farley
Software Developer
Red Hat