zephyr: enable progressive erase on all nRF boards

Enables BOOT_ERASE_PROGRESSIVELY for Nordic nRF boards.

This fixes issue where mcumgr upload command would result in
device being deleted, but no new image being uploaded
due to timeout while waiting for erase operation.

This avoids a delay of several seconds during firmware upload.

Signed-off-by: Bernt Johan Damslora <bernt.johan.damslora@nordicsemi.no>
Signed-off-by: Emil Obalski <emil.obalski@nordicsemi.no>
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
Signed-off-by: Håkon Øye Amundsen <haakon.amundsen@nordicsemi.no>
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>