Create the default layer regarding fields_desc dict

        :param field_pos_list:

def self_build(self, field_pos_list=None):
        """
        Create the default layer regarding fields_desc dict

        :param field_pos_list:
        """
        if self.raw_packet_cache is not None:
            for fname, fval in six.iteritems(self.raw_packet_cache_fields):
                if self.getfieldval(fname) != fval:
                    self.raw_packet_cache = None
                    self.raw_packet_cache_fields = None
                    self.wirelen = None
                    break
            if self.raw_packet_cache is not None:
                return self.raw_packet_cache
        p = b""
        for f in self.fields_desc:
            val = self.getfieldval(f.name)
            if isinstance(val, RawVal):
                sval = raw(val)
                p += sval
                if field_pos_list is not None:
                    field_pos_list.append((f.name, sval.encode("string_escape"), len(p), len(sval)))  # noqa: E501
            else:
                p = f.addfield(self, p, val)
        return p