Change the CC2530 and CC2538 default PAN ID to 0xABCD

When I was developing the CC2530 port back in the day, it felt like a good idea to use a different PAN ID than what was used by most other Contiki platforms.

In retrospect, it turned out to be a pretty bad idea: In order to knock together a testbed of CC2530s and other nodes, a user has to make changes to the code base - not great.

The same PAN ID was subsequently adopted by CC2538DK, with similar consequences.

With this pull, I am proposing restoring the default PAN ID to 0xABCD for both those platforms. This may be a little disruptive to some users in the first instance, but in the long term I think it will turn out to be a good move.

Shameless ping to the Cetic chaps (@laurentderu @sdawans), cause I think this particular situation has caused a fair bit of grief to 6lbr users and therefore they may want to keep this pull in their radar.