ffmpeg: fix packaging examples

ffmpeg: fix packaging examples

* I've noticed that ffmpeg package isn't created in my builds, due
  to --disable-programs in EXTRA_OECONF added by our .bbappend, but
  was surprised that lib32-ffmpeg is created.

* lib32-ffmpeg was created only because it contained the examples
  which are installed in /usr/share/ffmpeg even when PN is lib32-ffmpeg
  as we pass --datadir=${datadir}/ffmpeg in EXTRA_OECONF here

* --disable-programs controls ${bindir}/ffprobe ${bindir}/ffmpeg and
  ${datadir}/ffmpeg/ffprobe.xsd ${datadir}/ffmpeg/libvpx-*.ffpreset

Signed-off-by: Martin Jansa <martin.jansa@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>