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/runtime/runtime_rp2.go

@@ -7,7 +7,7 @@ import (
 	"device/rp"
 	"internal/task"
 	"machine"
-	"machine/usb/cdc"
+	_ "machine/usb/cdc"
 	"runtime/interrupt"
 	"runtime/volatile"
 	"unsafe"
@@ -360,8 +360,6 @@ func machineInit()
 func init() {
 	machineInit()
 
-	cdc.EnableUSBCDC()
-	machine.USBDev.Configure(machine.UARTConfig{})
 	machine.InitSerial()
 }