Credits to Helge Wagner:

grml has configured the net boot to always use eth0 ("conf/initramfs.conf": 
"DEVICE=eth0"). I have changed the "scripts\live" file to include a 
configuration option for the "DEVICE" variable:

                        ethdevice=*)
                                DEVICE="${ARGUMENT#ethdevice=}"
                                export DEVICE
                                ;;
                                                                                                                                                                                  
So now i can add e.g. "ethdevice=eth3" to the command line in my pxe config 
file. I tried that and now my system is happy (using eth3 to connect to the nfs 
server).

Note by mika: already contacted live-initramfs upstream.