here it is:

Obviously, in recent busyboxes, sourcing a non-existent file makes busybox
(silently?) exit, which in case of scripts/live leads to an immediate kernel
panic since the root fs is not yet mounted.

Wrapping the "./tmp/net-${device}.conf" into a

                if [ -e "/tmp/net-${device}.conf" ]; then
                  . /tmp/net-${device}.conf
                fi

in scripts/live solved the issue for me.

Greetings
Marc