os-prober ignores grub devicetree statement

[Juerg Haefliger]

os-prober doesn't recognize a grub devicetree statement which results in an unbootable grub entry.

I.e. from /boot/grub/grub.cfg on /dev/nvme0n1p5:

menuentry 'Ubuntu' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-ddc4f4b5-4f16-419d-aa64-50bbafcc93ae' {
        recordfail
        load_video
        gfxmode $linux_gfx_mode
        insmod gzio
        if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
        insmod part_gpt
        insmod ext2
        search --no-floppy --fs-uuid --set=root ddc4f4b5-4f16-419d-aa64-50bbafcc93ae
        linux /boot/vmlinuz-6.8.0-31-generic root=UUID=ddc4f4b5-4f16-419d-aa64-50bbafcc93ae ro clk_ignore_unused pd_ignore_unused arm64.nopauth
        initrd /boot/initrd.img-6.8.0-31-generic
        devicetree /boot/dtb-6.8.0-31-generic
}

But os-prober (running from /dev/nvme0n1p6) constructs the following (missing devictree line):

menuentry 'Ubuntu 23.10 (23.10) (on /dev/nvme0n1p5)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'osprober-gnulinux-simple-ddc4f4b5-4f16-419d-aa64-50bbafcc93ae' {
        insmod part_gpt
        insmod ext2
        search --no-floppy --fs-uuid --set=root ddc4f4b5-4f16-419d-aa64-50bbafcc93ae
        linux /boot/vmlinuz-6.8.0-31-generic root=UUID=ddc4f4b5-4f16-419d-aa64-50bbafcc93ae ro clk_ignore_unused pd_ignore_unused arm64.nopauth
        initrd /boot/initrd.img-6.8.0-31-generic
}