Fix the handling of 802.15.4 short addresses.

This includes 2 fixes for a proper handling of 802.15.4 short addresses:
- Generate an appropriate IPv6 link-local address when using a short address.
- Fix the mismatch between sizeof(rimeaddr_t) and UIP_LLADDR_LEN, both used in many parts of the network stack to get the size of the 802.15.4 addresses.