Two safety nets are added here:

  * PYTHON variable that can be set to the interpreter path (defaults to
    "python3"), so that the system default can be overriden
  * the PORT arg to usb-bootloader.py is passed as an empty string if
    unset, to produce an explicit error

Dealing with mixed integers and strings (or bytes() when pyserial
decides to do so) is a PITA that plagues all the code with ord()
conversions and makes it error-prone and hard to debug. It is even more
so if the python version (2.7 or 3.x) is not known in advance.

To play safe
    - the firmware file is converted at read time into a bytearray,
      which is what it is
    - all data flow that is supposed to come from/to the serial line is
      cast into integer values as soon/later as possible; given that
      pyserial gives back str/bytes somehow at will, ord() calls at the
      .recv() method level are the safest way to ensure we get-a-byte,
      get-a-byte, get-a-byte, byte, byte.

dev/simple_uart: re-enable FIFO & use macro for hardcoded baudrate calculation (saves 1kB of bootloader space...)