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/board_nucleof103rb.go

@@ -99,17 +99,16 @@ const (
 var (
 	// USART2 is the hardware serial port connected to the onboard ST-LINK
 	// debugger to be exposed as virtual COM port over USB on Nucleo boards.
-	// Both UART0 and UART1 refer to USART2.
-	UART0  = &_UART0
-	_UART0 = UART{
+	UART2  = &_UART2
+	_UART2 = UART{
 		Buffer: NewRingBuffer(),
 		Bus:    stm32.USART2,
 	}
-	UART2 = UART0
+	Serial = UART2
 )
 
 func init() {
-	UART0.Interrupt = interrupt.New(stm32.IRQ_USART2, _UART0.handleInterrupt)
+	UART2.Interrupt = interrupt.New(stm32.IRQ_USART2, _UART2.handleInterrupt)
 }
 
 // SPI pins