settings: (NVS) fetch sector size from driver

NVS back-end initialization should fetch size of the flash erase blocks
from the flash API instead of DT. This allows to work well when used
storage partition is not located in embedded memory.

NVS back-end sector multiplier configuration was set to 8K as
DT value for native posix targets
flash sector sizes is 1 B, while its flash driver supports 8k.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>