x86: fix touching bootloader "upgraded" mark

The mount point is "/tmp/boot", the path in the boot partition is
"/boot/grub/upgraded".

Origin of this mark b9c1cf16 ("x86: add preinit hook for bootloader
upgrade")

Fixes 32f675ca ("x86: fix grub-bios-setup fail during sysupgrade")

Ref: https://bugs.openwrt.org/index.php?do=details&task_id=3140
Reported-by: Philip Prindeville <philipp@redfish-solutions.com>
Signed-off-by: Yousong Zhou <yszhou4tech@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 /tmp/boot/grub/upgraded
+		&& touch /tmp/boot/boot/grub/upgraded
 
 		umount /tmp/boot
 	fi