Reorganize netfilter kernel modules and package nftables kernel support

Signed-off-by: Steven Barth <steven@midlink.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42596 3c298f89-4303-0410-b956-a3cf2f4a3e73
libnftnl: Add nftables libnftnl

Signed-off-by: Steven Barth <steven@midlink.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42597 3c298f89-4303-0410-b956-a3cf2f4a3e73
nftables: introduce experimental nftables

Signed-off-by: Steven Barth <steven@midlink.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42598 3c298f89-4303-0410-b956-a3cf2f4a3e73
netfilter: remove redundant CONFIG_IP_NF_IPTABLES

Signed-off-by: Steven Barth <steven@midlink.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42599 3c298f89-4303-0410-b956-a3cf2f4a3e73