ffmpeg: use arch and cpu instead of tune

ffmpeg currently uses BR2_GCC_TARGET_TUNE as the --cpu option
value. However, there are multiple architectures for which
BR2_GCC_TARGET_TUNE is not used. After inspecting the ffmpeg configure
script, we concluded that using the BR2_GCC_TARGET_CPU value if
available, or the BR2_GCC_TARGET_ARCH value as a fallback was the
appropriate behavior.

This allows to remove the reference to BR2_GCC_TARGET_TUNE, which is
one step towards the removal of this option.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>