RFR: 8310384: Add hooks for custom image creation [v2]

[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.

Erik Joelsson has updated the pull request incrementally with one additional commit since the last revision:

  Applied same changes to jre image logic for symmetry

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/14563/files
  - new: https://git.openjdk.org/jdk/pull/14563/files/29d28234..8314e733

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14563&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14563&range=00-01

  Stats: 4 lines in 1 file changed: 1 ins; 0 del; 3 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