Do not generate dhcp entry for loopback interface

This fixes a regression that was introduced in commit 52474b4c28303324.
The variable $interface is pointing towards /sys/class/net/lo,
at least until we overwrite the variable by the basename of
its network interface name path. Accordingly fix order for comparison.

Closes: https://github.com/grml/live-boot-grml/issues/9
Thanks: Mykola Malkov for the bug report