boot/zephyr: add nrfx watchdog kick during boot region copy

This fixes soft-bricks that we have seen as a result
of the bootloader being interrupted by the watchdog.

Signed-off-by: Emil Hammarstrom <emil.hammarstrom@assaabloy.com>
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>