buildroot

gst1-libav: switch to unbundled

It avoids the crazy in handling all of the ffmpeg options here as well,
and potentially avoids target code duplication, hence resulting in a
size reduction.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Thomas: extend help text as suggested by Arnout.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>