Revert "db845c/BoardConfig.mk: set bootv3 as the default" am: e85af9cf46 am: 2b52c79ebf

Original change: https://android-review.googlesource.com/c/device/linaro/dragonboard/+/1656128

Change-Id: I3dc038e7eee24366d7ae0fe247ceccdad6e0c2e5