net/dhcpv4: Ensure udp header checksum is computed correctly

The dhcpv4 client code builds ip and udp packets from scratch rather
than using the network stack to do the heavy lifting (why ?).

When it computes the udp checksum of each packet it builds it neglects
to clear any preexisting detritus from the checksum field.  The result
of this is that some packets will be built with correct checksums and
some will be built with incorrect checksums.

This is the underlying reason that the dhcp client often taken many
retransmissions and elapsed time before in order to acquire an IP
address.

Change-Id: Iebd1ed34e06f7f2e53d45f6d1555e22f48490287
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>