boot: Add rollback counter to unprotected list

This really should be protected, but as it is not currently, allow it in the
TLVs that are allowed to be unprotected.

Signed-off-by: David Brown <david.brown@linaro.org>

boot/bootutil/src/image_validate.c

@@ -368,6 +368,7 @@ static const uint16_t allowed_unprot_tlvs[] = {
      IMAGE_TLV_ENC_KW,
      IMAGE_TLV_ENC_EC256,
      IMAGE_TLV_ENC_X25519,
+     IMAGE_TLV_SEC_CNT,
      /* Mark end with ANY. */
      IMAGE_TLV_ANY,
 };