bootutil: consolidate helpers for old flash API
bootutil: consolidate helpers for old flash API

Move helpers which are needed for using flash_area_to_sectors() to the
end of file. This is just to keep things clean when we add support for
flash_area_get_sectors().

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

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