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/board_pinetime-devkit0.go

@@ -17,7 +17,7 @@ const (
 	LED3 = LCD_BACKLIGHT_LOW
 )
 
-var Serial = UART0
+var DefaultUART = UART0
 
 // UART pins for PineTime. Note that RX is set to NoPin as RXD is not listed in
 // the PineTime schematic 1.0: