Separate out interface up/down from Thread start/stop. (#278)

* Redefine otEnable/otDisable to just initialize/uninitialize the OpenThread stack.
* Remove otInit API since it is replaced by otEnable.
* Add otInterfaceUp/otInterfaceDown APIs to bring up/down IPv6 interface.
* Add otThreadStart/otThreadStop APIs to start/stop Thread protocol operation.
* Updated NCP implementation to utilize new APIs.

examples/apps/cli/main.c

@@ -37,7 +37,7 @@ void otSignalTaskletPending(void)
 int main(int argc, char *argv[])
 {
     PlatformInit(argc, argv);
-    otInit();
+    otEnable();
     otCliUartInit();
 
     while (1)