ffmpeg: disable for ARMv7-M

In the ffmpeg code, libavutil/arm/asm.S makes some assumption about the
ARM architecture for which the code is built. Only ARMv4, ARMv5, ARMv6
and ARMv7-A is supported. Due to this, object files built out of
ARM-optimized assembly code have the wrong architecture information,
causing a failure at link time.

Adding ARMv7-M support would be possible, but it doesn't exist yet in
ffmpeg, and it's pretty unlikely that ffmpeg will ever be needed on an
ARMv7-M platform, so this commit takes the simple approach of disabling
ffmpeg for ARMv7-M.

Fixes:

  http://autobuild.buildroot.net/results/ca4c67b093afd6f14349fcdc87b02e0480172e8c/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>