RFR: JDK-8202914: Let custom makefile override jmod intput dir locations

[Tim Bell]

Erik:

> We need to be able to override some input dir locations from a custom
> makefile extension.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8202914
>
> Webrev: http://javaweb.us.oracle.com/~ejoelsso/webrev/8202914/webrev.01/

Looks good.

Tim