Set MAC address based on hash of K60 UID, using FNET prefix.

Previously the firmware would have the same MAC address as it was based on compilation time, but now we set it from SIM_UID.
We use the [djb2 hash function](http://www.cse.yorku.ca/~oz/hash.html) as we don't know which bits of the UID are unique, and we do not have space for them all.