buildroot

package/ffmpeg: default to --cpu=generic for MIPS architecture

This option is needed to prevent this message from configure:

WARNING: unknown CPU. Disabling all MIPS optimizations.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas:
 - group with the other --cpu conditions so that only one is passed.
 - improve the comment.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>