Running "ifup eth0" if eth0 is configured for dhcp would make the
client try a little while and then give up. Thus, if the dhcp server
is currently down we end up without IP address. This happens after a
power failure, if the switch reboots faster than the dhcp server.

Also, avoid ifplugd, that won't help in this case (the link is up,
so no "link up" will happen later.

This new configuration works in all cases:
     - eth0 is configured with a static address in /etc/network/interfaces
     - dhcp-driven, and link goes down and then up
     - dhcp-driven, and server not ready at boot time
     - dhcp-driven and all fine since boot time
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>