dfu: img_util: Add flash integrity check

Flash memory can be write but there is no way to check flash integrity.
Add flash_img_check method that verify flash integrity.  This is useful
to avoid firmware reboot and test.  Another use is ensure that firmware
upgrade routines from internet server to flash slot are performing
properly.  This uses flash_area_check_int_sha256 method to check a
SHA-256 hash.  On sucess match, zero is returned, otherwise a negative
errno value.

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>