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 );