RFR: JDK-8235686: Add more custom hooks in Bundles.gmk

[Tim Bell]

Erik:

> We need to customize the image dirs in order to support the new Macos 
> signing requirements at Oracle. To support this, we need Bundles.gmk to 
> accept more variable overrides for where the various images are located.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8235686
> 
> Webrev: http://cr.openjdk.java.net/~erikj/8235686/webrev.01/

Looks good.

Tim