make CONF= isn't working

From the make help output:

Useful make variables
. make CONF= # Build all configurations (note, assignment is empty)
. make CONF=<substring> # Build the configuration(s) with a name matching
. # <substring>

But make CONF= isn't working:

erik@moya:jdk8-build$ make jdk CONF=
Building target 'jdk' in the following configurations (matching CONF=):
* linux-x86_64-normal-server-fastdebug
* linux-x86_64-normal-server-release
make: *** common/makefiles: Is a directory. Stop.
make: Nothing to be done for `jdk'.