machine: define Serial as the default output

Previously, the machine.UART0 object had two meanings:

  - it was the first UART on the chip
  - it was the default output for println

These two meanings conflict, and resulted in workarounds like:

  - Defining UART0 to refer to the USB-CDC interface (atsamd21,
    atsamd51, nrf52840), even though that clearly isn't an UART.
  - Defining NRF_UART0 to avoid a conflict with UART0 (which was
    redefined as a USB-CDC interface).
  - Defining aliases like UART0 = UART1, which refer to the same
    hardware peripheral (stm32).

This commit changes this to use a new machine.Serial object for the
default serial port. It might refer to the first or second UART
depending on the board, or even to the USB-CDC interface. Also, UART0
now really refers to the first UART on the chip, no longer to a USB-CDC
interface.

The changes in the runtime package are all just search+replace. The
changes in the machine package are a mixture of search+replace and
manual modifications.

This commit does not affect binary size, in fact it doesn't affect the
resulting binary at all.

src/machine/machine_atsamd21.go

@@ -505,8 +505,7 @@ type UART struct {
 }
 
 var (
-	// UART0 is actually a USB CDC interface.
-	UART0 = &USBCDC{Buffer: NewRingBuffer()}
+	USB = &USBCDC{Buffer: NewRingBuffer()}
 )
 
 const (
@@ -1985,7 +1984,7 @@ func handleUSB(intr interrupt.Interrupt) {
 
 	// Start of frame
 	if (flags & sam.USB_DEVICE_INTFLAG_SOF) > 0 {
-		UART0.Flush()
+		USB.Flush()
 		// if you want to blink LED showing traffic, this would be the place...
 	}
 
@@ -2045,7 +2044,7 @@ func handleUSB(intr interrupt.Interrupt) {
 				setEPINTFLAG(i, sam.USB_DEVICE_EPINTFLAG_TRCPT1)
 
 				if i == usb_CDC_ENDPOINT_IN {
-					UART0.waitTxc = false
+					USB.waitTxc = false
 				}
 			}
 		}
@@ -2359,7 +2358,7 @@ func handleEndpoint(ep uint32) {
 
 	// move to ring buffer
 	for i := 0; i < count; i++ {
-		UART0.Receive(byte((udd_ep_out_cache_buffer[ep][i] & 0xFF)))
+		USB.Receive(byte((udd_ep_out_cache_buffer[ep][i] & 0xFF)))
 	}
 
 	// set byte count to zero