Merge #681

681: Refactor 6LoWPAN function r=thvdveld a=thvdveld

Now, handling UDP packets is done using `process_udp`. Also removed `cfg_if!` in `interface` since that messed with formatting of the code.

It also contains a fix for the fragmentation.

Co-authored-by: Thibaut Vandervelden <thvdveld@vub.be>

Author: bors[bot]