Create a mechanism to modify the beacon version field. (#841)

* Create a mechanism to modify the beacon version field when the Join flag is set.

src/core/mac/mac_frame.hpp

@@ -34,6 +34,8 @@
 #ifndef MAC_FRAME_HPP_
 #define MAC_FRAME_HPP_
 
+#include <openthread-core-config.h>
+
 #include <limits.h>
 #include <stdint.h>
 #include <string.h>
@@ -799,7 +801,14 @@ public:
      * This method sets the Joining Permitted flag.
      *
      */
-    void SetJoiningPermitted(void) { mFlags |= kJoiningFlag; }
+    void SetJoiningPermitted(void) {
+        mFlags |= kJoiningFlag;
+
+#if OPENTHREAD_CONFIG_JOIN_BEACON_VERSION != kProtocolVersion
+        mFlags &= ~kVersionMask;
+        mFlags |=  OPENTHREAD_CONFIG_JOIN_BEACON_VERSION << kVersionOffset;
+#endif
+    }
 
     /**
      * This method returns a pointer to the Network Name field.

src/core/openthread-core-default-config.h

@@ -237,6 +237,16 @@
 #define OPENTHREAD_CONFIG_COAP_MAX_RETRANSMIT                   4
 #endif  // OPENTHREAD_CONFIG_COAP_MAX_RETRANSMIT
 
+/**
+ * @def OPENTHREAD_CONFIG_JOIN_BEACON_VERSION
+ *
+ * The Beacon version to use when the beacon join flag is set.
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_JOIN_BEACON_VERSION
+#define OPENTHREAD_CONFIG_JOIN_BEACON_VERSION                   kProtocolVersion
+#endif  // OPENTHREAD_CONFIG_JOIN_BEACON_VERSION
+
 /**
  * @def OPENTHREAD_CONFIG_LOG_LEVEL
  *