ffmpeg: let the user override the --cpu value

Grab the content of BR2_PACKAGE_FFMPEG_EXTRACONF after guessing the CPU,
so the user will still have the possibility to override it.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Reviewed-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>