DUE USB CDC: Do not send any character if no program on the PC is listening to them. This avoids Marlin waiting until the user actually opens a program that is able to consume the output of Marlin

Marlin/src/HAL/HAL_DUE/usb/usb_task.c

@@ -52,6 +52,7 @@
 
 static volatile bool main_b_msc_enable = false;
 static volatile bool main_b_cdc_enable = false;
+static volatile bool main_b_dtr_active = false;
 
 void HAL_idletask(void) {
   // Attend SD card access from the USB MSD -- Prioritize access to improve speed
@@ -69,7 +70,7 @@ void usb_task_msc_disable(void)               { main_b_msc_enable = false; }
 bool usb_task_msc_isenabled(void)             { return main_b_msc_enable; }
 
 bool usb_task_cdc_enable(const uint8_t port)  { return ((main_b_cdc_enable = true)); }
-void usb_task_cdc_disable(const uint8_t port) { main_b_cdc_enable = false; }
+void usb_task_cdc_disable(const uint8_t port) { main_b_cdc_enable = false; main_b_dtr_active = false; }
 bool usb_task_cdc_isenabled(void)             { return main_b_cdc_enable; }
 
 /*! \brief Called by CDC interface
@@ -87,12 +88,17 @@ void usb_task_cdc_config(const uint8_t port, usb_cdc_line_coding_t *cfg) {
     dwDTERate = cfg->dwDTERate;
 }
 
+
 void usb_task_cdc_set_dtr(const uint8_t port, const bool b_enable) {
+
+  // Keep DTR status
+  main_b_dtr_active = b_enable;
+
   //  Implement Arduino-Compatible kludge to enter programming mode from
   // the native port:
   //  "Auto-reset into the bootloader is triggered when the port, already
   // open at 1200 bps, is closed."
-    
+
   if (1200 == dwDTERate) {
     // We check DTR state to determine if host port is open (bit 0 of lineState).
     if (!b_enable)
@@ -102,6 +108,8 @@ void usb_task_cdc_set_dtr(const uint8_t port, const bool b_enable) {
   }
 }
 
+bool usb_task_cdc_dtr_active(void)             { return main_b_dtr_active; }
+
 /// Microsoft WCID descriptor
 typedef struct USB_MicrosoftCompatibleDescriptor_Interface {
   uint8_t bFirstInterfaceNumber;