storage/flash_map: Added function for get erased byte value

Added flash_area_erased_val() function for get value of erased
byte of memory which is under flash area.
This function already exist in MCUBoot and zephyr dfu subsystem
which makes simultaneous usage of both impossible.

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