ffmpeg: fix ARM condition for --{enable, disable}-vfp

As noted by Arnout, not all ARMv6 have the VFP FPU, therefore instead
of using a condition on ARMv6 or ARMv7-A, this commit changes to use a
condition on BR2_ARM_CPU_HAS_VFPV2.

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