When the NIB is compiled for 6LN mode (but not a 6LBR), the Stateless
Address Autoconfiguration (SLAAC) functionality is disabled, as it is
typically not required; see `sys/include/net/gnrc/ipv6/nib/conf.h`, ll.
46 and 55. However, if a non-6LN interface is also compiled in (still
without making the node a border router) an auto-configured address will
be assigned in accordance with [RFC 6775] to the interface, just
assuming the interface is a 6LN interface. As it then only performs
duplicate address detection RFC-6775-style then, the address then never
becomes valid, as the duplicate address detection according to [RFC
4862] (part of the SLAAC functionality) is never performed.
As auto-configuring an address without SLAAC doesn't make sense, this
fix makes the interface skip it completely, but provides a warning to
the user, so they know what to do.
[RFC 6775]: https://tools.ietf.org/html/rfc6775#section-5.2
[RFC 4862]: https://tools.ietf.org/html/rfc4862#section-5.4
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