[Lenny400] Better configuration for Ethernet

[Dr. H. Nikolaus Schaller]

Just a small note that there is a improvement for the Ethernet
interface startup.

Currently, the boot process waits for ifup eth0 to be finished
before a login appears. If no cable is plugged in, this needs 
quite a while (30-60 seconds) and prints some DHCP errors.

The solution is to replace 

auto eth0 

in /etc/network/interfaces with

allow-hotplug eth0