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/runtime/runtime_nrf.go

@@ -29,7 +29,7 @@ func main() {
 }
 
 func init() {
-	machine.UART0.Configure(machine.UARTConfig{})
+	machine.Serial.Configure(machine.UARTConfig{})
 	initLFCLK()
 	initRTC()
 }
@@ -64,7 +64,7 @@ func initRTC() {
 }
 
 func putchar(c byte) {
-	machine.UART0.WriteByte(c)
+	machine.Serial.WriteByte(c)
 }
 
 func sleepTicks(d timeUnit) {