endianness of serialized ints is undefined (compiler-dependent)

[ecc1]

In the serial.c code

uint16_t serial_rx_word() {
  return (serial_rx_byte() << 8) + serial_rx_byte();
}

the order of the calls to serial_rx_byte is undefined (up to the C compiler). Similarly for serial_rx_long. So the endianness is not well-defined.