Kevin,
These blogs might help explain things:
http://blogs.sun.com/kto/entry/anatomy_of_the_jdk_build
http://blogs.sun.com/jjg/entry/building_javac_for_jdk7
In jdk7/hotspot the BOOT jdk is used to do javac compilers, for some
serviceability tools,
but that should probably be changed to use the BOOT jdk java with the
javac.jar from the
langtools repository.
-kto