RFR: 8310384: Add hooks for custom image creation

[Erik Joelsson]

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, ahead 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.

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Commit messages:
 - JDK-8310384

Changes: https://git.openjdk.org/jdk/pull/14563/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14563&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310384
  Stats: 16 lines in 1 file changed: 6 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/14563.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14563/head:pull/14563

PR: https://git.openjdk.org/jdk/pull/14563