Many ffmpeg + rpi compilation guides on the internet recommend
using `./configure --enable-omx --enable-omx-rpi`. This fails
to find the IL OMX headers on device because the omx require_headers
check happens first before the add_cflags in omx_rpi.

A workaround is to use `./configure --enable-omx-rpi` only, since
omx_rpi already implies omx. But because many users expect to use
existing scripts and commands, we swap the order here so omx_rpi
special cases are applied first.

In the past this wasn't an issue because users noticed the OMX_Core.h
missing error and installed libomxil-bellagio-dev. But since
76c82843, the rpi specific headers from /opt/vc/include/IL
are required.
Signed-off-by: Aman Gupta <aman@tmm1.net>