Implement new McpsRetransmit primitive

The primitive will be invoked whenever the LoRaMac-node library
retransmits an uplink message. This could be used by the application to
get notified of uplink retransmissions.

src/mac/LoRaMac.c

@@ -3442,6 +3442,11 @@ LoRaMacStatus_t SendFrameOnChannel( uint8_t channel )
     // Send now
     Radio.Send( MacCtx.PktBuffer, MacCtx.PktBufferLen );
 
+    if(MacCtx.ChannelsNbTransCounter != 1 && MacCtx.MacPrimitives->MacMcpsRetransmit != NULL)
+    {
+        MacCtx.MacPrimitives->MacMcpsRetransmit( );
+    }
+
     return LORAMAC_STATUS_OK;
 }
 

src/mac/LoRaMac.h

@@ -2550,6 +2550,12 @@ typedef struct sLoRaMacPrimitives
      * \param   [OUT] MCPS-Confirm parameters
      */
     void ( *MacMcpsConfirm )( McpsConfirm_t* McpsConfirm );
+    /*!
+     * \brief   MCPS-Retransmit primitive
+     *
+     * \param   [OUT] MCPS-Retransmit parameters
+     */
+    void ( *MacMcpsRetransmit )( void );
     /*!
      * \brief   MCPS-Indication primitive
      *