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.

targets/esp32.json

@@ -3,6 +3,7 @@
 	"cpu": "esp32",
 	"build-tags": ["esp32", "esp"],
 	"scheduler": "tasks",
+	"serial": "uart",
 	"linker": "xtensa-esp32-elf-ld",
 	"default-stack-size": 2048,
 	"cflags": [