add ipv6 support

When you want to use ipv6 feature on rt-thread, you should do

1. Define RT_USING_LWIP_HEAD option in rtconfig.h configuration file. 
2. In rtconfig.h file, add the macro `#define RT_LWIP_IPV6 1` (to support ipv6). If you want to use automatical IPv6 address, you should add the macro `#define RT_LWIP_IPV6_AUTOCONFIG 1`. But firstly you should get a router which support ipv6(such as HIWIFI and other openwrt router).

3. If you want to use static IPv6 address, you can use setif() function in your application. For example, to set the "e0" device with "4006::1" address, you can use set_if6("e0", "4006::1") after network initialization;

Doing all of this, you can begin to use IPv6. 

How to test IPv6 function.

1. To ping board.
You can use "ping6 + linklocal_address + scope id" or "ping6 + ipv6 address" to ping board. 
In Linux, you can use "ping6 fe80::200:eff:fe12:3456%eth1"("fe80::200:eff:fe12:3456"is linklocal address of board)("eth1" is computer's link-local address scope id. 
In Windows 7 or Windows XP, you can find scope id through ipconfig command ); And "ping6 4006:e024:680:c6e:200:eff:fe12:3456" is also supported.("4006:e024:680:c6e:200:eff:fe12:3456" is ipv6 address of board).

2. BSD Socket API & netconn API
Please see the example in "apps/ipv6" directory. It contains the example of TCP server, TCP client, UDP server and UDP client, which are written with BSD Socket API.  TCP echo server, UDP echo server are written with netconn API. And I also write some ipv6 network debugging tools on linux platform， you can see here https://github.com/hduffddybz/IPV6_SOCKET