x86: fix grub-bios-setup fail during sysupgrade

grub-bios-setup requires two images (boot.img and core.img),
but they are missing. This make an error during sysupgrade:
Upgrading bootloader on /dev/sda...
grub-bios-setup: error: cannot open `/tmp/boot/boot/grub/boot.img': No
such file or directory.

Signed-off-by: 李国 <uxgood.org@gmail.com>

target/linux/x86/base-files/lib/upgrade/platform.sh

@@ -63,7 +63,7 @@ platform_do_bootloader_upgrade() {
 			-d "/tmp/boot/boot/grub" \
 			-r "hd0,${parttable}1" \
 			"/dev/$diskdev" \
-		&& touch /boot/grub/upgraded
+		&& touch /tmp/boot/grub/upgraded
 
 		umount /tmp/boot
 	fi