Split out definition of net_app_init() and its parameter flags from
net_app.h header to new net_config.h header. As we do this, rename
the function to net_config_init() and flags to NET_CONFIG_NEED_*.
This is a second step in splitting out network configuration API
out of net_app API, started in the c60df1311 commit.
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>