contrib: enable AMF encoder in FFMPEG contrib build

This draft adds AMD AMF (Advanced Media Framework) contrib package which is needed for enabling h264_amf and hevc_amf encoders in ffmpeg build. AMF hardware encoders are useful for transcoding/streaming and can be enabled in VLC settings by definition of codec name (i.e. "h264_amf") in "Encoding" section. Additional settings for the AMF encoder can be passed thru "Advanced options" at the moment. --enable-amf option can be passed into bootstrap command while building contrib packages.
By default, USE_AMF=1 is commented, but to be honest I'm not sure whether this solution is good for enabling hardware-specific features.