As reported by Boaz Nahum <boaznahum@gmail.com> on build-dev@openjdk.java.net:

make CONF=
Works just fine.

But when trying to add a target, it won't recognize it:

$ make CONF= images
Building target 'images' in the following configurations (matching CONF=):
* windows-x86-normal-server-release
* windows-x86_64-normal-server-release
make: *** No rule to make target `images'. Stop.