net: if: Add OpenThread interfaces unique name

OpenThread interfaces were not considered when default interface name
is assigned, so they ended up with a generic "netX" name. Since it
useful to have an option to identify interface easily by name, assign an
unique "threadX" name for OpenThread interfaces, just like it's done for
other interface types.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>