Add modules target in the forest's topdir Makefile

[Mandy Chung]

Kelly,

I'm adding a "modules" target in the topdir's Makefile that will pass 
the modules target to the jdk build.  This provides a convenience way to 
build the jdk module images.  In addition, the jprt build will archive 
the jdk-module-image and use it for testing.

Can you please review this change?
   http://cr.openjdk.java.net/~mchung/jigsaw/modules-target/

Thanks
Mandy