net: app: Allow to specify both static IP settings and DHCPv4.

The idea is that static config is used unless/until DHCP values
arrive. This allows to have the same network configuration values
for both a case of direct board - workstation connection (where
DHCP is usually not available), and a case where both a board and
workstation connect to a router (which serves DHCP).

The changes in this commit however take care of netmask and gateway
settings, but not about IP address itself. This is addressed in the
next patch.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>