RFR: JDK-8087193: Support building with devkits on Macosx

[Erik Joelsson]

Hello,

Please review this minor tweak to configure which enables the use of a 
portable devkit on Macosx. Also included is the script used to generate 
the devkit. The plan is use this internally on our build servers to ease 
installation and setup of our machines.

Bug: https://bugs.openjdk.java.net/browse/JDK-8087193
Webrev: http://cr.openjdk.java.net/~erikj/8087193/webrev.01/

/Erik