tfm: Add NS build support for zephyr out-of-tree nordic_nrf platform.

Add build of the NS application in the zephyr defined out-of-tree board
support for the nordic_nrf platform.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>
Signed-off-by: Markus Swarowsky <markus.swarowsky@nordicsemi.no>