drivers/flash: Add flash_fill() and flash_flatten()

The commit adds two new API calls:
 - flash_fill - that allows to fill selected part of device with
   specified value;
 - flash_flatten - that allows to erase or fill device with
   erase_value, depending on whether driver for the device provides
   erase callback.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>