net: dhcpv4: Accept unicast replies

Some DHCPv4 servers do not respect BROADCAST flag set on DHCP Discover,
replying with unicast packet, making it impossible to obtain DHCP
address by Zephyr in such cases.

RFC1542 chapter 3.1.1 makes the following statement about the BROADCAST
flag:
    This addition to the protocol is a workaround for old host
    implementations.  Such implementations SHOULD be modified so
    that they may receive unicast BOOTREPLY messages, thus making
    use of this workaround unnecessary.  In general, the use of
    this mechanism is discouraged.

Making it clear that being able to process unicast replies from the DHCP
server is not only an optional behavior, but a recommended solution.

Therefore, introduce a support for unicast DHCPv4 in Zephyr. To achieve
this, add additional filtering rule at the IPv4 level - in case DHCPv4
is enabled, there is an active query and the packet is destined for the
DHCPv4 module, let it through for the DHCPv4 module to process,
regardless of the destination IP address.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>