update_verifier: Set the success flag if dm-verity is not enabled.

For devices that are not using dm-verity, update_verifier can't verify
anything, but to mark the successfully booted flag unconditionally.

Test: Successfully-booted flag is set on devices w/o dm-verity.
Test: Successfully-booted flag is set after verification on devices w/
      dm-verity.
Change-Id: I79ab2caec2d4284aad0d66dd161adabebde175b6