Fix DHCP protocol so U-Boot does not respond too early

on the network with it's offered IP number; it should not reply until
after it has received a DHCP ACK message. Also ensures that U-Boot
does it's DHCPREQUEST as broadcast (per RFC 2131).

Signed-off-by: Robin Getz <rgetz@blackfin.uclinux.org>
Acked-by: Ben Warren <biggerbadderben@gmail.com>
Signed-off-by: Wolfgang Denk <wd@denx.de>