nd6: add nd6_clear_destination_cache() function

The new function, while currently not used internally, allows external
code to clear the ND destination cache in the case that it may have
become inconsistent with the current situation, for example as the
result of a change of locally assigned addresses, or a change in
routing tables implemented through the LWIP_HOOK_ND6_GET_GW hook.

src/core/ipv6/nd6.c

@@ -1293,6 +1293,22 @@ nd6_new_destination_cache_entry(void)
   return j;
 }
 
+/**
+ * Clear the destination cache.
+ *
+ * This operation may be necessary for consistency in the light of changing
+ * local addresses and/or use of the gateway hook.
+ */
+void
+nd6_clear_destination_cache(void)
+{
+  s8_t i;
+
+  for (i = 0; i < LWIP_ND6_NUM_DESTINATIONS; i++) {
+    ip6_addr_set_any(&destination_cache[i].destination_addr);
+  }
+}
+
 /**
  * Determine whether an address matches an on-link prefix.
  *