implementation of ND-6loWPAN [RFC 6775]

The commit of this pull is to update the neighbor discovery files to
implement the new optimized version of neighbor discovery which
suggested by IETF in RFC 6775.
IPv6 Neighbor Discovery (ND) based on RFC4861 was developed primarily
for wired traffic on the shared medium. Its heavily use the multicast
transmission in periodic router-advertisement multicast addresses with
router solicitation, neighbor solicitation, address resolution, and
duplicate-address detection. While periodic multicast signaling and
solicited-node multicast signaling are useful for network stability in
the standard Ethernet-based shared network, the limited-lifetime,
battery-operated devices in the IEEE 802.15.4 network conserve energy
with less signaling and by sending broadcast messages only once in a
while. That makes it inefficient and sometimes impractical for IPv6 over
Low power Wireless Personal Area Networks (6LoWPAN).
(https://tools.ietf.org/html/rfc6775) optimizes multicast messages to
uni-cast messages. It eliminates the need for relatively expensive
address resolution by sending a neighbor registration option along with
a neighbor solicitation; it supports sleepy nodes in the networks; and
it optimizes the protocol constants while eliminating periodic
router-advertisement messages.
You could find all details in the following paper
http://eece.cu.edu.eg/~akhattab/files/ND.pdf

update tcpip.c to handle ND timers

core/net/ip/tcpip.c

@@ -467,7 +467,7 @@ eventhandler(process_event_t ev, process_data_t data)
 #if !UIP_CONF_ROUTER
     if(data == &uip_ds6_timer_rs &&
         etimer_expired(&uip_ds6_timer_rs)) {
-      uip_ds6_send_rs();
+      uip_ds6_send_rs(NULL);
       tcpip_ipv6_output();
     }
 #endif /* !UIP_CONF_ROUTER */