db845c/BoardConfig.mk: set bootv3 as the default am: 63f8e16a54 am: 02fe7e8a56 am: 8b0a57fe17

Original change: https://android-review.googlesource.com/c/device/linaro/dragonboard/+/1652236

Change-Id: Ie4e59ecdc8102c3935813769e87a318cf6bb0dc1