Add serial initialization

src/machine/serial-none.go

@@ -5,3 +5,6 @@ package machine
 
 // Serial is a null device: writes to it are ignored.
 var Serial = NullSerial{}
+
+func InitSerial() {
+}

src/machine/serial-uart.go

@@ -5,3 +5,7 @@ package machine
 
 // Serial is implemented via the default (usually the first) UART on the chip.
 var Serial = DefaultUART
+
+func InitSerial() {
+	Serial.Configure(UARTConfig{})
+}

src/machine/serial-usb.go

@@ -5,3 +5,6 @@ package machine
 
 // Serial is implemented via USB (USB-CDC).
 var Serial = USB
+
+func InitSerial() {
+}

src/runtime/runtime_atsamd51.go

@@ -34,6 +34,7 @@ func init() {
 	//}
 
 	machine.InitUSB()
+	machine.InitSerial()
 }
 
 func putchar(c byte) {