update_verifier: Support androidboot.veritymode being empty or 'disabled'.

    // The current slot has not booted successfully.

#if defined(PRODUCT_SUPPORTS_VERITY) || defined(BOARD_AVB_ENABLE)

    bool skip_verification = false;

    std::string verity_mode = android::base::GetProperty("ro.boot.veritymode", "");

    if (verity_mode.empty()) {

      // With AVB it's possible to disable verification entirely and

      // in this case ro.boot.veritymode is empty.

#if defined(BOARD_AVB_ENABLE)

      LOG(WARNING) << "verification has been disabled; marking without verification.";

      skip_verification = true;

#else

      LOG(ERROR) << "Failed to get dm-verity mode.";

      return reboot_device();

#endif

    } else if (android::base::EqualsIgnoreCase(verity_mode, "eio")) {

      // We shouldn't see verity in EIO mode if the current slot hasn't booted successfully before.

      // Continue the verification until we fail to read some blocks.

      LOG(WARNING) << "Found dm-verity in EIO mode.";

    } else if (android::base::EqualsIgnoreCase(verity_mode, "disabled")) {

      LOG(WARNING) << "dm-verity in disabled mode; marking without verification.";

      skip_verification = true;

    } else if (verity_mode != "enforcing") {

      LOG(ERROR) << "Unexpected dm-verity mode : " << verity_mode << ", expecting enforcing.";

      return reboot_device();

    }

    if (!skip_verification) {

      static constexpr auto CARE_MAP_FILE = "/data/ota_package/care_map.txt";

      if (!verify_image(CARE_MAP_FILE)) {

        LOG(ERROR) << "Failed to verify all blocks in care map file.";

        return reboot_device();

      }

    }

#else

    LOG(WARNING) << "dm-verity not enabled; marking without verification.";

#endif