Revert "db845c/BoardConfig.mk: set bootv3 as the default" am: e85af9cf46 am: 2b52c79ebf am: 473591a6ad

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

Change-Id: I55d2ae67ce990cdc0ba059f7f44db3cbec0a06a7