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_esp8266.go

@@ -139,6 +139,8 @@ func (p Pin) PortMaskClear() (*uint32, uint32) {
 	return &esp.GPIO.GPIO_OUT_W1TC.Reg, 1 << p
 }
 
+var DefaultUART = UART0
+
 // UART0 is a hardware UART that supports both TX and RX.
 var UART0 = &_UART0
 var _UART0 = UART{Buffer: NewRingBuffer()}