Busybox init, Kivy python application appears to block execution of ifup

I am experimenting with Buildroot, trying to create a minimal system to optimise cold boot time into a Python application (Kivy GUI). I chose to use Busybox init process because this is supposed to be optimal for embedded systems. I have a Sxx script in /etc/init.d that starts my app: `#!/bin/sh python myapp.py 2 > errlog.txt &` This works when I pass loglevel=8 on kernel command line. The system boots into my Kivy app and I am able to ping/ssh into the Raspberry Pi2. However if I pass loglevel=1 then eth0 no longer comes up (everything else is ok). Numbering the startup script S99myapp so that it is executed after S40network does not affect the outcome. I also tried giving myapp low priority by starting it with nice -n 19 python myapp.py 2 > errlog.txt & but this again does not help. The problem is still there even with the simplest Kivy app possible - the 'Hello Word' example from Kivy home page. It looks like the Kivy app is somehow preventing ifup from completing, unless enough time is bought with printing log messages to console. Can someone please explain what is happening? Is there a way round this?