Pr/slaac utility (#698)

* Provide utility for IPv6 SLAAC (#651)

* Semantically Opaque Interface Identifier with IPv6 SLAAC (#651)

src/cli/cli.hpp

@@ -131,6 +131,7 @@ private:
     enum
     {
         kMaxArgs = 32,
+        kMaxAutoAddresses = 8,
     };
 
     void AppendResult(ThreadError error);
@@ -232,7 +233,7 @@ private:
     uint32_t sInterval;
     Timer sPingTimer;
 
-    otNetifAddress sAutoAddresses[8];
+    otNetifAddress mAutoAddresses[kMaxAutoAddresses];
 
     otInstance *mInstance;
 };