avoid multiple USB serial begin (taken from F1)

STM32F4/cores/maple/usb_serial.cpp

@@ -36,20 +36,25 @@
 #ifdef SERIAL_USB
 
 #define USB_TIMEOUT 50
+bool USBSerial::_hasBegun = false;
 
 USBSerial::USBSerial(void) {
 }
 
 void USBSerial::begin(void) {
+    if (_hasBegun)
+        return;
+    _hasBegun = true;
     setupUSB();
 }
 
 void USBSerial::begin(int) {
-    setupUSB();
+    this->begin();
 }
 
 void USBSerial::end(void) {
     disableUSB();
+	_hasBegun = false;
 }
 
 size_t USBSerial::write(uint8 ch) {

STM32F4/cores/maple/usb_serial.h

@@ -65,6 +65,9 @@ public:
 
     void enableBlockingTx(void);
     void disableBlockingTx(void);
+
+protected:
+    static bool _hasBegun;
 };
 
 extern USBSerial SerialUSB;