examples: rpl_udp cleanup

* removed unnecessary code
* simplified
* removed demo leftovers
* some minor fixes (use correct IPv6 address...)
Why not just
```C
ipv6_addr_init(&tmp, 0xabcd, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0);
```

then. It is semantically equivalent to your example.