net/dhcpv4: Correct backoff and retransmit behaviour

Implement RFC2131 4.1.1 requirement that at dhcp implementation wait
for a random delay of 1 to 10s before sending the initial DISCOVER.

Implement RF2131 4.1 requirement that at dhcp implementation set the
initial retransmit timeout at 4 seconds and exponentially backoff on
each retransmit.

Change-Id: Id7029f3ed16a5f886dbd555fed87320aeffe31aa
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>