examples: renamed rpl_udp header
examples: cleanup for rpl_udp

* removed unnecessary shell commands
* updated documentation
* Unified global default IP address
rpl: moved headers to sys/net/include
examples: don't advertise IPv6 prefix

Cleaning up the IPv6 address initialization and removed the addition of
an IPv6 prefix to advertise.
examples: use correct broadcast address when sending
fix low-level sending destination address
licenses: updated some missing licenses in headers
Merge pull request #1554 from OlegHahm/rpl_udp_cleanup

examples: rpl_udp cleanup