net: shell: Add information about IPv6 privacy extension

When executing "net iface" command, print current status of
IPv6 privacy extension if it is enabled in config file.

The "net ipv6 ..." command prints IPv6 privacy extension
information, and can add or delete IPv6 prefix filters.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>