Add hooks for custom image creation

We have a need for creating custom variants of the JDK image and would like to be able to reuse the existing Images.gmk for doing so. To be able to do that, we need to define some variables that are suitable for overrides. Specifically, we need:

EXTRA_MODULES: a list of extra java module names to include in the "jdk" image.

EXTRA_JMODS_DIR: a list of extra directories to add to the module path and look for jmods in, before the default jmods dir.

JDK_IMAGE_SUPPORT_DIR: override the location of the support dir. Without overriding this, multiple calls to Images.gmk will overwrite each others intermediate files and dependency files.