package/ffmpeg: bump version to 4.2

package/ffmpeg: bump version to 4.2

Updated license hash after three upstream commits:
http://git.videolan.org/?p=ffmpeg.git;a=history;f=LICENSE.md;h=c7d6bf7121d8811b36ee2a4ae6b810c072ed70e4;hb=refs/heads/release/4.2

Build-tested using this defconfig:
BR2_x86_64=y
BR2_x86_core2=y
BR2_TOOLCHAIN_EXTERNAL=y
BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD=y
BR2_TOOLCHAIN_EXTERNAL_URL="http://autobuild.buildroot.org/toolchains/tarballs/br-x86-64-core2-full-2019.05.1.tar.bz2"
BR2_TOOLCHAIN_EXTERNAL_GCC_7=y
BR2_TOOLCHAIN_EXTERNAL_HEADERS_5_1=y
BR2_TOOLCHAIN_EXTERNAL_LOCALE=y
BR2_TOOLCHAIN_EXTERNAL_CXX=y
BR2_PACKAGE_FFMPEG_NONFREE=y
BR2_PACKAGE_FFMPEG_FFPLAY=y
BR2_PACKAGE_FFMPEG_FFPROBE=y
BR2_PACKAGE_FFMPEG_POSTPROC=y
BR2_PACKAGE_GSTREAMER=y
BR2_PACKAGE_GST_FFMPEG=y
BR2_PACKAGE_GST_FFMPEG_GPL=y
BR2_PACKAGE_GSTREAMER1=y
BR2_PACKAGE_GST1_LIBAV=y
BR2_PACKAGE_LAME=y
BR2_PACKAGE_MOTION=y
BR2_PACKAGE_MPD=y
BR2_PACKAGE_MPD_FFMPEG=y
BR2_PACKAGE_MPV=y
BR2_PACKAGE_SQUEEZELITE=y
BR2_PACKAGE_VLC=y
BR2_PACKAGE_WAVPACK=y
BR2_PACKAGE_FREERDP=y
BR2_PACKAGE_QT5=y
BR2_PACKAGE_XORG7=y
BR2_PACKAGE_AUBIO=y
BR2_PACKAGE_FDK_AAC=y
BR2_PACKAGE_LIBMODPLUG=y
BR2_PACKAGE_LIBSOXR=y
BR2_PACKAGE_OPUS=y
BR2_PACKAGE_FONTCONFIG=y
BR2_PACKAGE_LIBDRM=y
BR2_PACKAGE_OPENCV3=y
BR2_PACKAGE_OPENCV3_WITH_FFMPEG=y
BR2_PACKAGE_OPENJPEG=y
BR2_PACKAGE_DAV1D=y
BR2_PACKAGE_LIBASS=y
BR2_PACKAGE_LIBBLURAY=y
BR2_PACKAGE_LIBOPENH264=y
BR2_PACKAGE_LIBTHEORA=y
BR2_PACKAGE_LIBYUV=y
BR2_PACKAGE_MEDIASTREAMER=y
BR2_PACKAGE_X265=y
BR2_PACKAGE_RTMPDUMP=y
BR2_PACKAGE_FETCHMAIL=y
BR2_PACKAGE_FREESWITCH=y
BR2_PACKAGE_GERBERA=y
BR2_PACKAGE_MINIDLNA=y
BR2_PACKAGE_TVHEADEND=y
BR2_PACKAGE_TVHEADEND_TRANSCODING=y

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