gst-ffmpeg: merge in bbappend from meta-oe

gst-ffmpeg: merge in bbappend from meta-oe

* Enable external libav for better optimisations/additional bugfixes
  (internal ffmpeg copy is quite old), default enabled but can be
  disabled using PACKAGECONFIG
* Add a PACKAGECONFIG for orc, disabled by default in line with other
  gstreamer recipes
* Bump PR to r7 so the bbappend can be dropped without PR going
  backwards

(From OE-Core rev: aab668fbba25d3e590e4182224b7b064d7705c5b)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>