Add a byte to the checksum.

def _add_to_checksum(self, checksum, value):
        """Add a byte to the checksum."""
        checksum = self._byte_rot_left(checksum, 1)
        checksum = checksum + value
        if (checksum > 255):
            checksum = checksum - 255
        self._debug(PROP_LOGLEVEL_TRACE, "C: " + str(checksum) + " V: " + str(value))
        return checksum