OK, I just look around a bit...
In /linuxrc (the first bootscript) is asking for the boot-device. So I need to set the boot_dev variable in the NVRAM.
Since the network isn't set in /linuxrc, it should be set in the debian-installation.

I think all I need is an working /etc/networking/interfaces in the debian distro (this is the config-place for debians network...) or an way to test the debian network-script.

PS. Getting debian on the WL-HDD is really ugly. Now I need to disconnect
the harddrive, unset the boot_dev variable, reboot, connect the harddrive again...

Is there an other option to unset the boot_dev variable?