Bluetooth: Mesh: enhanced provisioning authentication

Implementation of the enhanced provisioning authentication (EPA)
that is based on SHA256 and HMAC security algorithms.
OOB authentication mandates EPA usage.

Co-authored-by: Krzysztof Kopyściński <krzysztof.kopyscinski@codecoup.pl>
Signed-off-by: Aleksandr Khromykh <aleksandr.khromykh@nordicsemi.no>
Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>