[nat64] fix border router itself cannot use nat64 (#158)
[nat64] fix border router itself cannot use nat64 (#158)

* [fix-nat64-ping] fix border router itself cannot use nat64

* [fix-nat64-ping] nat64 interface does not need a global IPv6 address

openthread/ot-br-posix,openthread/ot-br-posix,jwhui/borderrouter,bukepo/borderrouter,openthread/borderrouter,openthread/borderrouter,bukepo/borderrouter,openthread/borderrouter,openthread/ot-br-posix,bukepo/borderrouter,bukepo/borderrouter,jwhui/borderrouter,openthread/ot-br-posix,jwhui/borderrouter,bukepo/borderrouter,openthread/ot-br-posix,openthread/borderrouter,openthread/borderrouter,jwhui/borderrouter,openthread/ot-br-posix,openthread/borderrouter,jwhui/borderrouter