ffmpeg: add conflicts and provides

ffmpeg: add conflicts and provides

Motivation of this change is that full variants provides the mini
variant and as well audio-dec package, thus you can not install both as
it fails with the following output:

Collected errors:
 * check_data_file_clashes: Package libffmpeg-audio-dec wants to install file /usr/lib/libavcodec.so.58
	But that file is already provided by package  * libffmpeg-full
 * check_data_file_clashes: Package libffmpeg-audio-dec wants to install file /usr/lib/libavcodec.so.58.91.100
	But that file is already provided by package  * libffmpeg-full
 * check_data_file_clashes: Package libffmpeg-audio-dec wants to install file /usr/lib/libavdevice.so.58
	But that file is already provided by package  * libffmpeg-full
 * check_data_file_clashes: Package libffmpeg-audio-dec wants to install file /usr/lib/libavdevice.so.58.10.100
	But that file is already provided by package  * libffmpeg-full
 * check_data_file_clashes: Package libffmpeg-audio-dec wants to install file /usr/lib/libavformat.so.58
	But that file is already provided by package  * libffmpeg-full
 * check_data_file_clashes: Package libffmpeg-audio-dec wants to install file /usr/lib/libavformat.so.58.45.100
	But that file is already provided by package  * libffmpeg-full
 * check_data_file_clashes: Package libffmpeg-audio-dec wants to install file /usr/lib/libavutil.so.56
	But that file is already provided by package  * libffmpeg-full
 * check_data_file_clashes: Package libffmpeg-audio-dec wants to install file /usr/lib/libavutil.so.56.51.100
	But that file is already provided by package  * libffmpeg-full
 * opkg_install_cmd: Cannot install package libffmpeg-audio-dec.

Let's change it to:
Installing libffmpeg-audio-dec (4.3.4-1) to root...
Collected errors:
 * check_conflicts_for: The following packages conflict with libffmpeg-audio-dec:
 * check_conflicts_for:         libffmpeg-full *
 * opkg_install_cmd: Cannot install package libffmpeg-audio-dec.

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>