brcm63xx: create initramfs HCS images

Cable Modem devices cannot load an ELF file directly so we need to
provide them with a HCS image even when targetting initramfs.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@36954 3c298f89-4303-0410-b956-a3cf2f4a3e73