package/tvheadend: Add transcoding support by ffmpeg

Add --disable-libffmpeg_static to disable static linking of ffmpeg
libraries into the tvheadend binary.

tvheadend compiles with ffmpeg using this minimal defconfig:

BR2_TOOLCHAIN_BUILDROOT_LARGEFILE=y
BR2_TOOLCHAIN_BUILDROOT_INET_IPV6=y
BR2_PACKAGE_FFMPEG=y
BR2_PACKAGE_TVHEADEND=y

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