NcpBase: Implement get handler for IPV6_LL_ADDR spinel property (#1826)

This commit add a new API `otThreadGetLinkLocalIp6Address()` to
get the Thread link-local address (which is is derived using
IEEE802.15.4 Extended Address as Interface Identifier).  The new
API is then used to implement the get handler for spinel property
`IPV6_LL_ADDR` in `NcpBase` class.

src/core/api/thread_api.cpp

@@ -195,6 +195,11 @@ exit:
     return error;
 }
 
+const otIp6Address *otThreadGetLinkLocalIp6Address(otInstance *aInstance)
+{
+    return &aInstance->mThreadNetif.GetMle().GetLinkLocalAddress();
+}
+
 const char *otThreadGetNetworkName(otInstance *aInstance)
 {
     return aInstance->mThreadNetif.GetMac().GetNetworkName();