db845c/BoardConfig.mk: set bootv3 as the default

enable vendor_boot by default for the db845c build

Test: boot tested with the bootv3 images

Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
Change-Id: I792eefc4b825cb936ccd0d942fe2b94457d1e06c
1 file changed