Add CONFIG_NET_DHCPV4, which, after https://github.com/zephyrproject-rtos/zephyr/pull/5750 works as follows: static addresses are configured after boot, and DHCP requests are sent at the same time. If valid DHCP reply is received, it overrides static addresses. This setup works out of the box for both direct connection to a workstation (DHCP server usually is not available) and for connection to a router (DHCP is available and required).