net/dhcpv4: Use BROADCAST and UNICAST appropriately.

RFC2131 places explict requirements on a client w.r.t which messages
are broadcast and which are unicast directly to a server.  Notable
rules as they apply to the current dhcpv4 implementation are that
DISCOVER and a REQUEST in response to an OFFER are broadcast.  A
REQUEST in state RENEWAL is unicast.  There are further rules relevant
to the REBINDING state which is not yet implementated.

Adjust the current implementation that always uses broadcast to use
unicast as required by RFC2131

Change-Id: I6edef4241bcd74623a804a73415888cd679888d0
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>