ffmpeg: explicitly disable NEON support

When BR2_ARM_CPU_HAS_NEON=y, we pass --enable-neon. However, when
BR2_ARM_CPU_HAS_NEON is disabled, we don't pass anything. This generally
works fine, but turned out to integrate NEON code in ARMv7-M
builds (since it's ARMv7, ffmpeg assumed it should enable NEON code).

Even though ffmpeg is now disabled for ARMv7-M, it still makes sense to
be explicit, and disable NEON support when the CPU doesn't have it.

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