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

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

Change-Id: I3e9a048a413994a89a8d841ee7366297f0ff6214