brcm63xx: make use of the new BuildDTB command

Now that we have a BuildDTB command, make use of it instead of calling
dtc directly.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46390 3c298f89-4303-0410-b956-a3cf2f4a3e73