gnrc_ipv6_nib: don't auto-configure IPv6 w/o SLAAC on non-6LN interface

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

sys/net/gnrc/network_layer/ipv6/nib/_nib-slaac.c

@@ -15,6 +15,7 @@
 
 #include <stdbool.h>
 
+#include "log.h"
 #include "luid.h"
 #include "net/gnrc/netif/internal.h"
 
@@ -34,6 +35,15 @@ void _auto_configure_addr(gnrc_netif_t *netif, const ipv6_addr_t *pfx,
     int idx;
     uint8_t flags = GNRC_NETIF_IPV6_ADDRS_FLAGS_STATE_TENTATIVE;
 
+#if !GNRC_IPV6_NIB_CONF_SLAAC
+    if (!gnrc_netif_is_6ln(netif)) {
+        LOG_WARNING("SLAAC not activated; will not auto-configure IPv6 address "
+                         "for interface %u.\n"
+                    "    Use GNRC_IPV6_NIB_CONF_SLAAC=1 to activate.\n",
+                    netif->pid);
+        return;
+    }
+#endif
     if (!(netif->flags & GNRC_NETIF_FLAGS_HAS_L2ADDR)) {
         DEBUG("nib: interface %i has no link-layer addresses\n", netif->pid);
         return;