image.mk: use XZ bcj architecture filter if available

xz supports using an architecture specific BCJ filter, use that one to
save us a couple kB while producing squashfs filesystems (mimics the
Linux kernel).

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35097 3c298f89-4303-0410-b956-a3cf2f4a3e73