hg: build-infra/jdk8: Implemented 'overlay-images' and added support for them in new compare script.

[erik.joelsson at oracle.com]

Changeset: b15491505102
Author:    erikj
Date:      2012-08-30 15:42 +0200
URL:       http://hg.openjdk.java.net/build-infra/jdk8/rev/b15491505102

Implemented 'overlay-images' and added support for them in new compare script.

! common/autoconf/compare.sh.in
! common/autoconf/spec.gmk.in
! common/makefiles/Makefile