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/README.md

@@ -23,10 +23,17 @@ $ cd <path-to-openthread>/output/<platform>/bin
 $ ./ot-cli 1
 ```
 
-Start OpenThread:
+Bring up the IPv6 interface:
 
 ```bash
-> start
+> ifconfig up
+Done
+```
+
+Start Thread protocol operation:
+
+```bash
+> thread start
 Done
 ```
 
@@ -58,10 +65,17 @@ $ cd <path-to-openthread>/output/<platform>/bin
 $ ./ot-cli 2
 ```
 
-Start OpenThread:
+Bring up the IPv6 interface:
 
 ```bash
-> start
+> ifconfig up
+Done
+```
+
+Start Thread protocol operation:
+
+```bash
+> thread start
 Done
 ```
 

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)