package/ffmpeg: Bump to version 2.3.1

- remove obsolete option --disable-vis
- add patch to fix compile error with uClibc
- add patch to fix compile error in freerdp

Compile-tested against all packages depending on ffmpeg using this defconfig:

BR2_TOOLCHAIN_EXTERNAL=y
BR2_PACKAGE_GSTREAMER=y
BR2_PACKAGE_GST_FFMPEG=y
BR2_PACKAGE_MPD=y
BR2_PACKAGE_MPD_FFMPEG=y
BR2_PACKAGE_XORG7=y
BR2_PACKAGE_FREERDP=y
BR2_PACKAGE_OPENCV=y
BR2_PACKAGE_OPENCV_WITH_FFMPEG=y
BR2_PACKAGE_MEDIASTREAMER=y
BR2_PACKAGE_MINIDLNA=y

VLC also compiled when the build of vlc-static is disabled in Makefiles,
otherwise this known error occurs:
http://autobuild.buildroot.net/results/022/022252176c051ff3365b2cb820f32aafc802803a/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>