Improve how OpenJ9.mk builds the images

[yongja79]

The issue to get jlink to work #21 brought up how OpenJ9.gmk builds the build jdk, image jdk and image jre and how it should be improved.

Is closed/OpenJ9.gmk is the correct place for this. The [pre/post] Images.gmk might be a better location or extending 'module'-libs targets or maybe customizing/extending jdk/make/lib/Lib-java.base.gmk?

If closed/OpenJ9.gmk is the right place, improve the macro used to copy across the J9 files into the images. See if a single macro is possible and if removing the explicit copies can be done. See PR #66 for more information.

[JamesKingdon]

We also need to consider how the build jdk should be handled for cross compiling, as that currently doesn't seem to work.

[andrew-m-leonard]

this is now obsolete, as the make file has been reworked quite a bit since then now