brcm63xx: switch to new image building system

Switches BCM63xx to the new IB system.
Images are generated per device definitions.
There are no changes in the produced images except for some name changes,
caused by the new IB system.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46490 3c298f89-4303-0410-b956-a3cf2f4a3e73