add board_reset_to_bootloader(), try to implement that for ch32v203 but not working yet

examples/device/cdc_dual_ports/src/main.c

@@ -119,6 +119,26 @@ static void cdc_task(void) {
   }
 }
 
+// Invoked when cdc when line state changed e.g connected/disconnected
+// Use to reset to DFU when disconnect with 1200 bps
+void tud_cdc_line_state_cb(uint8_t instance, bool dtr, bool rts) {
+  (void)rts;
+
+  // DTR = false is counted as disconnected
+  if (!dtr) {
+    // touch1200 only with first CDC instance (Serial)
+    if (instance == 0) {
+      cdc_line_coding_t coding;
+      tud_cdc_get_line_coding(&coding);
+      if (coding.bit_rate == 1200) {
+        if (board_reset_to_bootloader) {
+          board_reset_to_bootloader();
+        }
+      }
+    }
+  }
+}
+
 //--------------------------------------------------------------------+
 // BLINKING TASK
 //--------------------------------------------------------------------+