netfilter: Make legacy configs user selectable

This option makes legacy Netfilter Kconfig user selectable, giving users
the option to configure iptables without enabling any other config.

Make the following KConfig entries user selectable:
 * BRIDGE_NF_EBTABLES_LEGACY
 * IP_NF_ARPTABLES
 * IP_NF_IPTABLES_LEGACY
 * IP6_NF_IPTABLES_LEGACY

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>