Update to the coding conventions regarding names of standardized constants and variables

Dear all,

FYI: I amended the coding convention with a statement about names in standardization documents (e.g. RFCs) [1]. It’s a no-brainer: They should be used at least in Doxygen to allow for easy reference when cross-checking (or review) the implementation of the standard.

Cheers, Martine

[1] https://github.com/RIOT-OS/RIOT/wiki/Coding-conventions#naming