brcm63xx: use dtb for cvg834g

Add a dtsi for bcm3368, add a dts and profile for cvg834g, and convert
it to use ImageDTB. Since HCS requires more arguments, enhance the max
arguments of the call.
The image name is intentionally left blank to prevent non-initramfs
images to be built, as they currently contain no rootfs and consequently
won't work.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44561 3c298f89-4303-0410-b956-a3cf2f4a3e73