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/serial.go

@@ -0,0 +1,46 @@
+package machine
+
+import "errors"
+
+var errNoByte = errors.New("machine: no byte read")
+
+// UARTConfig is a struct with which a UART (or similar object) can be
+// configured. The baud rate is usually respected, but TX and RX may be ignored
+// depending on the chip and the type of object.
+type UARTConfig struct {
+	BaudRate uint32
+	TX       Pin
+	RX       Pin
+}
+
+// NullSerial is a serial version of /dev/null (or null router): it drops
+// everything that is written to it.
+type NullSerial struct {
+}
+
+// Configure does nothing: the null serial has no configuration.
+func (ns NullSerial) Configure(config UARTConfig) error {
+	return nil
+}
+
+// WriteByte is a no-op: the null serial doesn't write bytes.
+func (ns NullSerial) WriteByte(b byte) error {
+	return nil
+}
+
+// ReadByte always returns an error because there aren't any bytes to read.
+func (ns NullSerial) ReadByte() (byte, error) {
+	return 0, errNoByte
+}
+
+// Buffered returns how many bytes are buffered in the UART. It always returns 0
+// as there are no bytes to read.
+func (ns NullSerial) Buffered() int {
+	return 0
+}
+
+// Write is a no-op: none of the data is being written and it will not return an
+// error.
+func (ns NullSerial) Write(p []byte) (n int, err error) {
+	return len(p), nil
+}