machine: only enable USB-CDC when needed

Enabling USB consumes a lot of power. So it's better to keep it disabled
by default when the serial port is set to something other than "usb"
(USB-CDC).

On my nice!nano clone, it reduces current consumption from 1mA to
0.13mA (and most of the rest is probably consumed by a connected SCD41
sensor).

This change might break some expectations, when a board uses USB but not
USB-CDC (I think this is rare, but I didn't check specifically).

src/machine/machine_esp32c3.go

@@ -519,6 +519,10 @@ type Serialer interface {
 	RTS() bool
 }
 
+func initUSB() {
+	// nothing to do here
+}
+
 // USB Serial/JTAG Controller
 // See esp32-c3_technical_reference_manual_en.pdf
 // pg. 736

src/machine/serial-usb.go

@@ -6,5 +6,6 @@ package machine
 var Serial Serialer
 
 func InitSerial() {
+	initUSB()
 	Serial = USBCDC
 }

src/machine/usb.go

@@ -19,6 +19,17 @@ var (
 	USBCDC Serialer
 )
 
+func initUSB() {
+	enableUSBCDC()
+	USBDev.Configure(UARTConfig{})
+}
+
+// Using go:linkname here because there's a circular dependency between the
+// machine package and the machine/usb/cdc package.
+//
+//go:linkname enableUSBCDC machine/usb/cdc.EnableUSBCDC
+func enableUSBCDC()
+
 type Serialer interface {
 	WriteByte(c byte) error
 	Write(data []byte) (n int, err error)