When doing `dhcp`, there is a bad dhcp server in my network
which always reply dhcp request with yiaddr 0, which cause
uboot can not successfully get ipaddr from the good dhcp server.
But the Linux PC can get the ip address even if there is a bad
dhcp server. This patch is to fix that even if there is a bad
dhcp server, uboot can still get ipaddr and tftp work ok.

The way is to ignore the packets from the bad dhcp server by filtering
out the yiaddr whose value is 0.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Wolfgang Denk <wd@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>