include: util: Add `util_eq` and `util_memeq`

`util_eq` compare two memory areas and their length. It returns true if
they are equal, else false.

`util_memeq` the first n bytes of two memory areas. It returns true if
they are equal, else false.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>