net: rpl: Add default interface type setting to Kconfig.rpl

Allow user to define what kind of network the RPL should serve.
Currently options will be either ANY or IEEE802154. If there
is only one network interface in the system, then ANY will take
the default network interface and use that. If there are multiple
network interfaces, then one should not use ANY as the default
network interface might not be the expected one.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>