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

[Erik Joelsson]

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/

/Erik