AP_Scripting: clear serial device sim buffers appropriately

Ensures the script won't process data created before it started, and
that the protocol won't process data created after the script stopped.

libraries/AP_Scripting/AP_Scripting.cpp

@@ -311,6 +311,11 @@ void AP_Scripting::thread(void) {
             GCS_SEND_TEXT(MAV_SEVERITY_CRITICAL, "Scripting: %s", "Unable to allocate memory");
             _init_failed = true;
         } else {
+#if AP_SCRIPTING_SERIALDEVICE_ENABLED
+            // clear data in serial buffers that the script wasn't ready to
+            // receive
+            _serialdevice.clear();
+#endif
             // run won't return while scripting is still active
             lua->run();
 
@@ -345,6 +350,11 @@ void AP_Scripting::thread(void) {
             }
         }
 #endif // AP_NETWORKING_ENABLED
+
+#if AP_SCRIPTING_SERIALDEVICE_ENABLED
+        // clear data in serial buffers that hasn't been transmitted
+        _serialdevice.clear();
+#endif
         
         // Clear blocked commands
         {

libraries/AP_Scripting/AP_Scripting_SerialDevice.cpp

@@ -35,6 +35,13 @@ void AP_Scripting_SerialDevice::init(void)
     }
 }
 
+void AP_Scripting_SerialDevice::clear(void)
+{
+    for (auto &p : ports) {
+        p.clear();
+    }
+}
+
 /*
   initialise port
  */
@@ -43,6 +50,17 @@ void AP_Scripting_SerialDevice::Port::init(void)
     begin(1000000, 0, 0); // assume 1MBaud rate even though it's a bit meaningless
 }
 
+void AP_Scripting_SerialDevice::Port::clear(void)
+{
+    WITH_SEMAPHORE(sem);
+    if (readbuffer) {
+        readbuffer->clear();
+    }
+    if (writebuffer) {
+        writebuffer->clear();
+    }
+}
+
 size_t AP_Scripting_SerialDevice::Port::device_write(const uint8_t *buffer, size_t size)
 {
     WITH_SEMAPHORE(sem);

libraries/AP_Scripting/AP_Scripting_SerialDevice.h

@@ -19,12 +19,14 @@ public:
     AP_Int8 enable;
 
     void init(void);
+    void clear(void);
 
 public:
     class Port : public AP_SerialManager::RegisteredPort {
     public:
         friend class AP_Scripting_SerialDevice;
         void init(void);
+        void clear(void);
 
         size_t device_write(const uint8_t *buffer, size_t size);
         ssize_t device_read(uint8_t *buffer, uint16_t count);