use bytes (integers) all along

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.