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.
I think we don't need neither `malloc` nor VLAs here. Basically, `rpl_routing_table` there are currently three possibilities for the required size of this array:
1. n - 2 for all nodes in storing mode
1. 1 for all non-root nodes in non-storing mode
1. n (for the worst case) for the root node

I think it reasonable to define at build time if a node has enough memory to either join a RPL instance in storing mode or act as root node (since this is basically defined by the overall available memory of the node). Hence, you could allocate the memory in a static matter.