Merge pull request ARMmbed#1876 from ARMmbed/thread_dhcpv6_server_trace_fix

Added trace about prefered timeout for fix CI test

Author: Juha Heiskanen

source/6LoWPAN/Thread/thread_dhcpv6_server.c

@@ -53,6 +53,7 @@ static void thread_dhcp_address_prefer_remove_cb(int8_t interfaceId, uint8_t *ta
         //Clear All targets routes
         ipv6_route_table_remove_info(interfaceId, ROUTE_THREAD_PROXIED_HOST,prefix_info);
     } else {
+        tr_debug("Address Preferred Timeout");
         ipv6_route_delete(targetAddress, 128, interfaceId, NULL, ROUTE_THREAD_PROXIED_HOST);
         thread_service_remove_GUA_from_neighcache(curPtr, targetAddress);