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.
Some time ago, back in FeuerWare days, there was something like a application `config.h` that could be used for these purposes. We could re-introduce something like that. But that's really a different discussions. For this PR, I would suggest to leave it as is and have a separate discussion about this.