[nrf fromlist] imgtool: create image obj with image_hash

Kept creating the field for external usage.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>

scripts/imgtool/image.py

@@ -604,6 +604,8 @@ def create(self, key, public_key_format, enckey, dependencies=None,
         sha.update(self.payload)
         digest = sha.digest()
         tlv.add(hash_tlv, digest)
+        # for external usage
+        self.image_hash = digest
         # Unless pure, we are signing digest.
         message = digest