replace all occurences of LINUX_VERSION with the cleaner
approach. future kernel upgrades must mostly touch only
one file. the only platform left is netlogic, because it
uses a intermediate kernel 3.14.16

Signed-off-by: Bastian Bittorf <bittorf@bluebottle.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43047 3c298f89-4303-0410-b956-a3cf2f4a3e73