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/feather-m4.json

@@ -1,6 +1,7 @@
 {
     "inherits": ["atsamd51j19a"],
     "build-tags": ["feather_m4"],
+    "serial": "usb",
     "flash-1200-bps-reset": "true",
     "flash-method": "msd",
     "msd-volume-name": "FEATHERBOOT",