Introduction of RPL non-storing mode

This is the first commit for discussion. It works - but e.g. the checksum in UDP-packages is wrong because the length is zero (which it is not btw). In addition the design decisions concerning messages sending/relaying may be discuss-able - but I think they are as noninvasive as the network stack would allow them to be.

At last - the proposed fixes concerning storing/non-storing mode files with the same declaration-base in one directory from @Kijewski does not work. (Both - excluding files or moving them to a sub-directory) But I will see more into it. Maybe someone can help.
So you suggest that a node has a pre-defined routing table even if it don´t need it? Its not in our vision of rpl yet, but a role in the network may change over time, so also the memory consumption. 

Anyway - to do it statically I can rewrite the init of a node, because than it has to be predefined if a node is root or normal node. Now there is first a general init and after that an optional root_init. Or you mean when I make the project, it should be already pre-defined if a node is always root or normal node?