kirkwood: add 3.18 support

Signed-off-by: Claudio Leite <leitec@staticky.com>
Signed-off-by: Luka Perkov <luka@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43613 3c298f89-4303-0410-b956-a3cf2f4a3e73