Sys: Adding defgroup to rpl.h

[kushalsingh007]

• Adding @defgroup and brief section to rpl.h
• Part of fix for RPL headers need documentation #2656

[kushalsingh007]

Since I wasn't exactly sure what the exact content should be I thought opening a PR and having your feedback would be a better idea.
@gebart :Can you list out the one's that you were referring to in #2656 i.e the one's for which rpl headers are completely missing Doxygen comments,and the one's which have Doxygen commands, but inside normal comment blocks. I plan to make all those minor changes in this PR.

[miri64]

According to #2422 this should be net_rpl. I also would remove the "Header". The other protocol implementations don't use it either: http://riot-os.org/api/group__net.html

[jnohlgard]

@kushalsingh007

• sys/net/include/rpl/rpl_dodag.h is missing everything
• sys/net/include/rpl/rpl_of_manager.h has broken @file documentation
• sys/net/include/rpl/rpl_structs.h is missing documentation for the data structures
• sys/net/include/rpl/rpl_config.h is missing documentation for the macros and data types
• sys/net/routing/rpl/of_mrhof.h has a filename after @file and has a closing brace (@}) for the @ingroup command placed too early, should be moved to near the end of the file.
• sys/net/routing/rpl/of_of0.h has a filename after @file and is missing documentation

Edit: Added some more files

[cgundogan]

@kushalsingh007 are you still working on this?

[kushalsingh007]

@cgundogan : I do have some of these in my local repo , however for those which need entire documentation to be written, I am afraid I might not be the best person to write documentation for those one's. As I do not have the idea about those one's in the big picture.

Still, I think I'll try to put here whatever I have done soon ( once my exams end in 2 days), Thanks for the reminder :). I am sure I'll need some help from you guys though.

[cgundogan]

I close this PR, because it references the former rpl implementation