Allow the 802.15.4/6lowpan code to operate on packets larger than 256 bytes

The old version of the 802.15.4 standard required packets to be <= 127 bytes long and the current Contiki 802.15.4 and 6lowpan code is hard-coded with this limit in mind. But for newer versions of the 802.15.4 standard (802.15.4g) that supports larger frames and for radios that allow sending and receiving larger radio frames than 127 bytes, this code cannot be used.

This patch removes this limitation so that the code can be used with packets larger than what an 8-bit byte can hold.