Radio packet length

Fixes #313.
Well, the problem that I see, is that this data goes actually over the air. Meaning, if we have a cc1100 and a, let's say, cc3200 with a maximum packet length of 1500 bytes, `radio_packet_length_t` will be `uint16_t`. That would lead to two bytes being transfered as a length field for the cc1100 as well, which in turn leads to an invalid packet.