examples: rpl_udp cleanup

* removed unnecessary code
* simplified
* removed demo leftovers
* some minor fixes (use correct IPv6 address...)
Can I use the same data struct to set the prefix? Something like:
```C
/* initialize prefix */
+ ipv6_addr_init(&tmp, 0xabcd, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, id);
ipv6_addr_init_prefix(&tmp, &tmp, 64);
```