Refactor OpenThread Global Variables into a Single Context Structure (#357)

* Refactor OpenThread to contain all global/static variables in a single context structure.

src/core/net/ip6.cpp

@@ -43,6 +43,7 @@
 #include <net/netif.hpp>
 #include <net/udp6.hpp>
 #include <thread/mle.hpp>
+#include <openthread-instance.h>
 
 namespace Thread {
 namespace Ip6 {
@@ -831,5 +832,10 @@ exit:
     return rval;
 }
 
+otInstance *Ip6::GetInstance()
+{
+    return otInstanceFromIp6(this);
+}
+
 }  // namespace Ip6
 }  // namespace Thread