ext: hal: nordic: cleanup Kbuild for now

Since nothing is currently using the nordic HAL lets just stick a dummy
build line.  Once something uses it, it can add what's needed and pull
in the code that is desired.

Change-Id: Id70bb6a88c6f1bbb7f33421cd0c14aafb83597b5
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>