bootutil: Remove arbitrary limit on BOOT_MAX_IMG_SECTORS

Count is initialized before it is passed to flash_area_get_sectors. The
flash driver should use count to ensure an overrun does not occur.

Signed-off-by: Kristine Jassmann <kristine.jassmann@renesas.com>

Signed-off-by: Gustavo Henrique Nihei <gustavo.nihei@espressif.com>