Previously WisunBorderRouter start() used NetworkInterface type for mesh interface, although only
WisunInterface type is possible for the call. Added a new overloads of the start with the
WisunInterface as mesh interface type and deprecated the old ones. This makes the calls stricter
about the interface type and safer. It also allows to remove the reinterpret_cast that causes
compiler warning on ARM compiler.
Added configuration functions and .json configuration options for:
- external RADIUS server IPv6 address
- RADIUS shared secret.
- RADIUS client retry trickle timer configuration. This can be used to set how fast
the RADIUS client retries Access-Request messages to RADIUS server in case reply
from server is not received.