nordic_nrf: Move partition include dir specification to board

So it is more easily overrideable. Now, an external platform can
specify their own partitions easily, and still use the core
nrf53/91 platforms.

Change-Id: Ia04d94e9ad4dc276f3b5d5cb61eeb13785cdb63d
Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>