Implement LoRaMac::disconnect

In order to reset LoRaMac's state in disconnect, we need to implement
an API which can be used to cancel all outstanding requests and reset
LoRaMac's internal state to idle.

This commit introduces LoRaMac::disconnect() which can be used for
this purpose.

Author: Kimmo Vaisanen

features/lorawan/LoRaWANStack.cpp

@@ -1193,6 +1193,9 @@ lorawan_status_t LoRaWANStack::lora_state_machine()
              */
             drop_channel_list();
 
+            // Shutdown LoRaMac
+            _loramac.disconnect();
+
             // Stop sending messages and set joined status to false.
 #if defined(LORAWAN_COMPLIANCE_TEST)
             _loramac.LoRaMacStopTxTimer();

features/lorawan/lorastack/mac/LoRaMac.cpp

@@ -1834,6 +1834,36 @@ lorawan_status_t LoRaMac::LoRaMacInitialization(loramac_primitives_t *primitives
     return LORAWAN_STATUS_OK;
 }
 
+void LoRaMac::disconnect()
+{
+    // Cancel all timers
+    _lora_time.TimerStop(_params.timers.mac_state_check_timer);
+    _lora_time.TimerStop(_params.timers.tx_delayed_timer);
+    _lora_time.TimerStop(_params.timers.rx_window1_timer);
+    _lora_time.TimerStop(_params.timers.rx_window2_timer);
+    _lora_time.TimerStop(_params.timers.ack_timeout_timer);
+
+    // Put radio to sleep
+    lora_phy->put_radio_to_sleep();
+
+    // Reset internal state
+    _params.is_nwk_joined = false;
+    _params.is_ack_retry_timeout_expired = false;
+    _params.is_rx_window_enabled = true;
+    _params.is_node_ack_requested = false;
+    _params.is_srv_ack_requested = false;
+    _params.flags.value = 0;
+    _params.mac_state = 0;
+
+    // Clear MAC commands
+    mac_commands.ClearCommandBuffer();
+    mac_commands.ClearRepeatBuffer();
+    mac_commands.ClearMacCommandsInNextTx();
+
+    // Set internal state to idle.
+    _params.mac_state = LORAMAC_IDLE;
+}
+
 lorawan_status_t LoRaMac::LoRaMacQueryTxPossible( uint8_t size, loramac_tx_info_t* txInfo )
 {
     AdrNextParams_t adrNext;

features/lorawan/lorastack/mac/LoRaMac.h

@@ -91,6 +91,14 @@ class LoRaMac
                                           LoRaPHY *phy,
                                           events::EventQueue *queue);
 
+    /*!
+     * \brief   Disconnect LoRaMac layer
+     *
+     * \details Cancels all outstanding requests and sets LoRaMac's
+     *          internal state to idle.
+     */
+    void disconnect(void);
+
     /*!
      * \brief   Queries the LoRaMAC whether it is possible to send the next frame with
      *          a given payload size. The LoRaMAC takes the scheduled MAC commands into