change length field size in packet_t to two bytes

this work is based on the idea of having a transceiver which can send more than 2^8 bytes. Currently the nativenet transceiver is able to send those packets. 

This PR lacks a patch for other code witch uses the transceiver and/or the packet_t type.

Do not merge until all code can handle a two bytes length field.
Let's start a discussion about this topic!