ext: Integrate Nordic's 802.15.4 radio driver into Zephyr

This patch includes the new files in the build and refactors the
Kconfig and Kbuild files in ext/hal/nordic to acommodate for the presence of
the radio driver.

Change-Id: Ifeda1f6d51916c7096be3c09ef7db6ca59c87728
Signed-off-by: Wojciech Bober <wojciech.bober@nordicsemi.no>