Configure IPv6 kernel options in config/Config-kernel.in

Configure IPv6 kernel options in config/Config-kernel.in

Revision 46834 changed IPv6 support from a module to builtin. But
since the configuration of the IPv6 kernel options was left in
package/kernel/linux/modules/netsupport.mk, this means that an
empty kmod-ipv6 module was still being generated (not packaged).

This patch moves the configuration of the IPv6 kernel options to
config/Config-kernel.in to remove this last bit of the module.

Note that CONFIG_IPV6_PRIVACY was dropped (enabled by default
since Linux v3.13), so this option is no longer needed.

See https://github.com/torvalds/linux/commit/5d9efa7ee99eed58388f186c13cf2e2a87e9ceb4

Signed-off-by: Arjen de Korte <arjen+openwrt@de-korte.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@48132 3c298f89-4303-0410-b956-a3cf2f4a3e73