h264_omx and non-existing PPS 0 referenced
It turns out it is impossible to live stream with the RPi hardware acceleration using ffmpeg.
For ffmpeg to configure the hardware acceleration to issue SPS/PPS frames during the stream and not only at the beginning of it, it needs to use a specific API of the raspberry firmware which it does not at the moment. 

Raspivid and gstreamer does that though. So turn to thoses solution and forget about ffmpeg.