Change API to require all calls be from the same context. (#78)

* Remove atomic driver and all uses of otPlatAtomic* in OpenThread.
* Change posix example to only use a single thread of execution.

src/cli/cli_udp.hpp

@@ -80,7 +80,7 @@ public:
 private:
     enum
     {
-        kMaxLineLength = 80,
+        kMaxLineLength = 128,
     };
 
     static void HandleUdpReceive(void *aContext, otMessage aMessage, const otMessageInfo *aMessageInfo);