netfilter.mk: extend nf_add macro to take a version dependency expression

	- nf_add now takes an optional 4th argument which specifies a kernel version dependency, e.g. "lt 3.7.0"
	- remove CompareKernelPatchVer conditionals around nf_add invocations, use version depends instead
	- fixes xt_LOG.ko packaging with Linux 3.6.0 and later

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@34681 3c298f89-4303-0410-b956-a3cf2f4a3e73