gnrc_netif: distinct is_6lo() and is_6ln()

The functions now are semantic distinct:

- gnrc_netif_is_6lo(): the interface is a 6Lo interface
- gnrc_netif_is_6ln(): the interface is using Neighbor Discovery
  according to RFC 6775

sys/include/net/gnrc/netif/internal.h

@@ -334,26 +334,29 @@ static inline bool gnrc_netif_is_rtr_adv(const gnrc_netif_t *netif)
  *
  * @attention   Requires prior locking
  * @note        Assumed to be true, when @ref GNRC_NETIF_NUMOF == 1 and
- *              @ref net_gnrc_sixlowpan module is included (and
- *              @ref GNRC_IPV6_NIB_CONF_6LN is not 0, otherwise assumed to be
- *              false).
+ *              @ref net_gnrc_sixlowpan module is included. When the
+ *              @ref net_gnrc_sixlowpan module is not included, it is assumed
+ *              to be false.
  *
  * @param[in] netif the network interface
  *
  * @return  true, if the interface represents a 6LN
  * @return  false, if the interface does not represent a 6LN
  */
-#define gnrc_netif_is_6lo(netif)    gnrc_netif_is_6ln(netif)
+#if ((GNRC_NETIF_NUMOF > 1) && defined(MODULE_GNRC_SIXLOWPAN)) || defined(DOXYGEN)
+bool gnrc_netif_is_6lo(const gnrc_netif_t *netif);
+#elif (GNRC_NETIF_NUMOF == 1) && defined(MODULE_GNRC_SIXLOWPAN)
+#define gnrc_netif_is_6lo(netif)                (true)
+#else
+#define gnrc_netif_is_6lo(netif)                (false)
+#endif
 
 /**
  * @brief   Checks if the interface represents a 6Lo node (6LN) according to
  *          RFC 6775
  *
  * @attention   Requires prior locking
- * @note        Assumed to be true, when @ref GNRC_NETIF_NUMOF == 1 and
- *              @ref net_gnrc_sixlowpan module is included (and
- *              @ref GNRC_IPV6_NIB_CONF_6LN is not 0, otherwise assumed to be
- *              false).
+ * @note        Assumed to be false, when @ref GNRC_IPV6_NIB_CONF_6LN is 0.
  *
  * @param[in] netif the network interface
  *
@@ -362,10 +365,11 @@ static inline bool gnrc_netif_is_rtr_adv(const gnrc_netif_t *netif)
  * @return  true, if the interface represents a 6LN
  * @return  false, if the interface does not represent a 6LN
  */
-#if (GNRC_NETIF_NUMOF > 1) || !defined(MODULE_GNRC_SIXLOWPAN) || defined(DOXYGEN)
-bool gnrc_netif_is_6ln(const gnrc_netif_t *netif);
-#elif GNRC_IPV6_NIB_CONF_6LN
-#define gnrc_netif_is_6ln(netif)                (true)
+#if GNRC_IPV6_NIB_CONF_6LN || defined(DOXYGEN)
+static inline bool gnrc_netif_is_6ln(const gnrc_netif_t *netif)
+{
+    return (netif->flags & GNRC_NETIF_FLAGS_6LN);
+}
 #else
 #define gnrc_netif_is_6ln(netif)                (false)
 #endif

sys/net/gnrc/netif/gnrc_netif.c

@@ -1093,8 +1093,8 @@ static ipv6_addr_t *_src_addr_selection(gnrc_netif_t *netif,
 }
 #endif  /* MODULE_GNRC_IPV6 */
 
-#if (GNRC_NETIF_NUMOF > 1) || !defined(MODULE_GNRC_SIXLOWPAN)
-bool gnrc_netif_is_6ln(const gnrc_netif_t *netif)
+#if (GNRC_NETIF_NUMOF > 1) && defined(MODULE_GNRC_SIXLOWPAN)
+bool gnrc_netif_is_6lo(const gnrc_netif_t *netif)
 {
     switch (netif->device_type) {
 #ifdef MODULE_GNRC_SIXLOENC
@@ -1110,7 +1110,7 @@ bool gnrc_netif_is_6ln(const gnrc_netif_t *netif)
             return false;
     }
 }
-#endif  /* (GNRC_NETIF_NUMOF > 1) || !defined(MODULE_GNRC_SIXLOWPAN) */
+#endif  /* (GNRC_NETIF_NUMOF > 1) && defined(MODULE_GNRC_SIXLOWPAN) */
 
 static void _update_l2addr_from_dev(gnrc_netif_t *netif)
 {