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