LZ4 compression is now available with 3.12 kernels offer it as an
option.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38534 3c298f89-4303-0410-b956-a3cf2f4a3e73