Add the endianness to the format

def set_endianess(self, pkt):
        """Add the endianness to the format"""
        end = self.endianess_from(pkt)
        if isinstance(end, str) and end:
            if isinstance(self.fld, UUIDField):
                self.fld.uuid_fmt = (UUIDField.FORMAT_LE if end == '<'
                                     else UUIDField.FORMAT_BE)
            else:
                # fld.fmt should always start with a order specifier, cf field
                # init
                self.fld.fmt = end[0] + self.fld.fmt[1:]