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?

Not exactly. I suggest to decide on the size of the buffer to store the routing table at compile time.

> Or you mean when I make the project, it should be already pre-defined if a node is always root or normal node?

Right! I think the RPL RFCs make assumptions about the root node being more powerful memory-wise at several points. So, while assigning the role of a node within a RPL network at run-time might be more flexible, I guess, the specification not really expects this and it's totally fine for us to predefine the role at compile time.