iptables: Turn ipv6 and libnfnetlink support into PACKAGECONFIG

iptables: Turn ipv6 and libnfnetlink support into PACKAGECONFIG

Detection of libnfnetlink is automatic in configure which means
that when you have meta-networking in your cosmos, it would create
a race condition where if libnfnetlink is already staged then it
will be enabled otherwise disabled. The issue happens quite often
with sstate and high parallelism. Since the dependency libnfnetlink
is not part of OE-Core, this patch turns it into a PACKAGECONFIG
which is diabled by default and iptables is patched to provide
the knob.

If you want to enable libnfnetlink support then it can be done
in a bbappend where you are sure that you are also including
meta-networking in your distro.

While at it also turned ipv6 support into packageconfig

(From OE-Core rev: 0332551d90c866c5874529e81819b81b534e14be)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>