omap: update to linux 3.18, it has been tested by at least one builder and...

omap: update to linux 3.18, it has been tested by at least one builder and 3.13 has not been built by buildbot in a long time

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44802 3c298f89-4303-0410-b956-a3cf2f4a3e73