dropbear: ensure the interface has an ip-address

Use network_get_ipaddrs_all to get all ip-addresses of an interface. If the
function fails, the interface does not exists or has not any suiteable ip
addresses assigned.

Use the returned ip-address(es) to construct the dropbear listen address.

Signed-off-by: Mathias Kresin <openwrt@kresin.me>

SVN-Revision: 42857