samd21,samd51: fix usbcdc initialization when -serial=uart (#2631)

machine/samd21,samd51: fix usbcdc initialization when -serial=uart by using machine.USB.Configured()

src/machine/machine_atsamd21.go

@@ -1742,6 +1742,7 @@ type USBCDC struct {
 	waitTxc           bool
 	waitTxcRetryCount uint8
 	sent              bool
+	configured        bool
 }
 
 var (
@@ -1923,6 +1924,13 @@ func (usbcdc *USBCDC) Configure(config UARTConfig) {
 	// enable IRQ
 	intr := interrupt.New(sam.IRQ_USB, handleUSB)
 	intr.Enable()
+
+	usbcdc.configured = true
+}
+
+// Configured returns whether usbcdc is configured or not.
+func (usbcdc *USBCDC) Configured() bool {
+	return usbcdc.configured
 }
 
 func handlePadCalibration() {

src/machine/machine_atsamd51.go

@@ -1965,6 +1965,7 @@ type USBCDC struct {
 	waitTxc           bool
 	waitTxcRetryCount uint8
 	sent              bool
+	configured        bool
 }
 
 var (
@@ -2149,6 +2150,13 @@ func (usbcdc *USBCDC) Configure(config UARTConfig) {
 	interrupt.New(sam.IRQ_USB_SOF_HSOF, handleUSBIRQ).Enable()
 	interrupt.New(sam.IRQ_USB_TRCPT0, handleUSBIRQ).Enable()
 	interrupt.New(sam.IRQ_USB_TRCPT1, handleUSBIRQ).Enable()
+
+	usbcdc.configured = true
+}
+
+// Configured returns whether usbcdc is configured or not.
+func (usbcdc *USBCDC) Configured() bool {
+	return usbcdc.configured
 }
 
 func handlePadCalibration() {

src/runtime/runtime_atsamd21.go

@@ -30,6 +30,9 @@ func init() {
 
 	// connect to USB CDC interface
 	machine.Serial.Configure(machine.UARTConfig{})
+	if !machine.USB.Configured() {
+		machine.USB.Configure(machine.UARTConfig{})
+	}
 }
 
 func putchar(c byte) {

src/runtime/runtime_atsamd51.go

@@ -30,6 +30,9 @@ func init() {
 
 	// connect to USB CDC interface
 	machine.Serial.Configure(machine.UARTConfig{})
+	if !machine.USB.Configured() {
+		machine.USB.Configure(machine.UARTConfig{})
+	}
 }
 
 func putchar(c byte) {