net: ip: dhcpv4: Handle DHCPV4_OPTIONS_DNS_SERVER

Actually set Zephyr's default DNS server based on the corresponding
DHCP option received. This makes DHCP-based setup Zephyr complete:
now it's possible to connect Zephyr DHCP-enabled system to a typical
router, and it will fully auto-configure to access Internet.

This initial implementation uses just first DNS server address as
returned in DHCP message, it may need to be extended in the future
based on the need.

Signed-off-by: John Andersen <john.s.andersen@intel.com>
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>