This finishes refactor of splitting off net_config library name from
net_app library, started in c60df1311, c89a06dbc. This commit makes
sure that Kconfig options are prefixed with CONFIG_NET_CONFIG_
instead of CONFIG_NET_APP_, and propagates these changes thru the
app configs in the tree.
Also, minor dependency, etc. tweaks are made.
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>