* gendistlist.sh (EXTRA_DISTFILES): Add `genvideolist.sh'.
        * genmk.rb (video): New variable.
        (CLEANFILES, VIDEOFILES): Add #{video}.
        (#{video}): New target rule.
        * genvideolist.sh: New file.
        * Makefile.in (pkglib_DATA): Add video.lst.
        (video.lst): New target rule.
        * util/grub-mkconfig.in: Initialize ${GRUB_VIDEO_BACKEND} using
        `video.lst'.
        * util/grub.d/30_os-prober.in: Replace `vbe' with
        ${GRUB_VIDEO_BACKEND}.