include: modify kernel.mk to build empty kmod packages if the referenced symbols do not exist in the config

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37400 3c298f89-4303-0410-b956-a3cf2f4a3e73
linux: add missing netfilter dependencies

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37401 3c298f89-4303-0410-b956-a3cf2f4a3e73
linux: introduce new kmod-iptunnel for Linux 3.10+, add missing dependencies to netsupport packages

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37402 3c298f89-4303-0410-b956-a3cf2f4a3e73