flash_map: declare flash_area_get_sectors()
flash_map: declare flash_area_get_sectors()

The current flash_map.h API treats flash areas and flash sectors
synonymously.  This isn't accurate; a flash area comprises one or more
sectors.

To distinguish them, add a new struct flash_sector, and a new
flash_area_get_sectors() which initializes an array of struct
flash_sector instead of struct flash area.

That done, deprecate flash_area_to_sectors().

Signed-off-by: Marti Bolivar <9d90ac4c7bf6a305f6bfd81a23c7859bc883380e@linaro.org>

tamban01/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,utzig/mcuboot,utzig/mcuboot,runtimeco/mcuboot,utzig/mcuboot,utzig/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,utzig/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,tamban01/mcuboot