These functions are independent from GNRC and can be used by stack-agnistoc
code.
Avoid pulling in a GNRC dependency by moving those two helper functions to
`netif`.
The old function names are kept as `static inline` wrapper functions to avoid
breaking API users.
In 77a7aed6e6bb51a248fa826585e837abf1df58a2 the API moved from an
identifier-based approach to a pointer based approach. The documentation
does not reflect this yet.