zephyr: Use flash_area_flatten in bs_custom_storage_erase

The intention of bs_custom_storage_erase is to remove data from device;
to support devices that do not require erase, without calling erase,
so that devices that do not implement such functions could work,
the flash_area_erase has been replaced with flash_area_flatten.

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