iptables: use nft backend with libnftnl PACKAGECONFIG

iptables: use nft backend with libnftnl PACKAGECONFIG

Currently, when the libnftnl (part of meta-networking) PACKAGECONFIG is
enabled for iptables, both legacy and nft-based binaries are built and
installed in the image. However, the "iptables" symlink in this case
still points to xtables-legacy-multi, rather than xtables-nft-multi.
This patch adds a conditional check to replace the symlink to point to
the latter if iptables is built with libnftnl support, which is
consistent with other major distros (e.g. Fedora). The "iptables-legacy"
symlink remains present and unmodified.

Signed-off-by: tgamblin <trevor.gamblin@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>