The omxil/iomx modules have higher priority than avcodec now
(since b79c09c on the main vlc repository), so instead of
explicitly asking for iomx when enabled, specify --codec
avcodec,all when it shouldn't be active.

If the omxil/iomx modules support other codecs that also are
provided by some other sw decoder than avcodec, those sw decoders
should also be listed here.