ifup: do not dhcp on network interface of secondary stack

Configure cmdline to:
 ip=br0:dhcp bridge=br0:bond0 bond=bond0:eth0

By default ifup bond0 will run dhcp on bond0, which is wrong. bond0
isn't the top interface. we should really run dhcp on br0.

So if we ifup an network interface on secondary stack, we should not
dhcp. Fix this issue with this patch.