zephyr/prj_base.conf: Enable DHCP and group static IPs together.

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).

ports/zephyr/prj_base.conf

@@ -32,16 +32,22 @@ CONFIG_NET_APP_SETTINGS=y
 CONFIG_NET_APP_INIT_TIMEOUT=3
 CONFIG_NET_APP_NEED_IPV6=y
 CONFIG_NET_APP_NEED_IPV4=y
-CONFIG_NET_APP_MY_IPV6_ADDR="2001:db8::1"
-CONFIG_NET_APP_MY_IPV4_ADDR="192.0.2.1"
-CONFIG_NET_APP_MY_IPV4_GW="192.0.2.2"
 
 # DNS
 CONFIG_DNS_RESOLVER=y
 CONFIG_DNS_RESOLVER_ADDITIONAL_QUERIES=2
 CONFIG_DNS_SERVER_IP_ADDRESSES=y
+
+# Static IP addresses
+CONFIG_NET_APP_MY_IPV6_ADDR="2001:db8::1"
+CONFIG_NET_APP_MY_IPV4_ADDR="192.0.2.1"
+CONFIG_NET_APP_MY_IPV4_GW="192.0.2.2"
 CONFIG_DNS_SERVER1="192.0.2.2"
 
+# DHCP configuration. Until DHCP address is assigned,
+# static configuration above is used instead.
+CONFIG_NET_DHCPV4=y
+
 # Diagnostics and debugging
 
 # Required for zephyr.stack_analyze()