net: app: allow IPv6 interface init to work with late IP assigment

Some network interfaces such as bluetooth 6lowpan can start without
an assigned IP address and then later once the connection is up a
router advertisement broadcast will assign the IP address.

The net_app framework will timeout out during network init if a value
cannot be parsed from CONFIG_NET_APP_MY_IPV6_ADDR.  Let's adjust
the framework to handle a missing value there and fill it in later
when the IPv6 address is added to the interface.

Signed-off-by: Michael Scott <michael.scott@linaro.org>