add ipv6 support

When you trying to use ipv6 on rt-thread, you should do
1、Change the SConscript file in components/net  , use RT_USING_LWIP_H to reply RT_USING_LWIP_141 or other.
2、in rtconfig.h file, add the macro #define RT_LWIP_IPV6		1, if you want to set the ipv6 address automatically , you should add the macro #define RT_LWIP_IPV6_AUTOCONFIG		1, but you should get a router that support ipv6(such as hiwifi and openwrt router) firstly.
3、if you want to set ipv6 address manually, in your application files , you can use setif() function, for example, you want to set the "e0" device of "fe80::1", you can use set_if6("e0", "fe80::1");
  Do all of this, you can use ipv6 happily.
And then, you will test it, use "ping6+linklocaladdress+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 scope id, in Windows 7 and 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).
 If you want to test BSD socket API and netconn API, please see the example in "apps", and the net test tools in computer, you can see here https://github.com/hduffddybz/IPV6_SOCKET