all: add a flag to the command line to select the serial implementation

This can be very useful for some purposes:

  * It makes it possible to disable the UART in cases where it is not
    needed or needs to be disabled to conserve power.
  * It makes it possible to disable the serial output to reduce code
    size, which may be important for some chips. Sometimes, a few kB can
    be saved this way.
  * It makes it possible to override the default, for example you might
    want to use an actual UART to debug the USB-CDC implementation.

It also lowers the dependency on having machine.Serial defined, which is
often not defined when targeting a chip. Eventually, we might want to
make it possible to write `-target=nrf52` or `-target=atmega328p` for
example to target the chip itself with no board specific assumptions.

The defaults don't change. I checked this by running `make smoketest`
before and after and comparing the results.

src/machine/machine_generic.go

@@ -11,6 +11,12 @@ var (
 	USB   = &UART{100}
 )
 
+// The Serial port always points to the default UART in a simulated environment.
+//
+// TODO: perhaps this should be a special serial object that outputs via WASI
+// stdout calls.
+var Serial = UART0
+
 const (
 	PinInput PinMode = iota
 	PinOutput
@@ -118,12 +124,6 @@ type UART struct {
 	Bus uint8
 }
 
-type UARTConfig struct {
-	BaudRate uint32
-	TX       Pin
-	RX       Pin
-}
-
 // Configure the UART.
 func (uart *UART) Configure(config UARTConfig) {
 	uartConfigure(uart.Bus, config.TX, config.RX)