x86: grub2: search for the "kernel" filesystem on all disks

Previously, grub2 was hardcoded to always look on "hd0" for the
kernel.

This works well when the system only had a single disk.
But if there was a second disk/stick present, it may have look
on the wrong drive because of enumeration races.

This patch utilizes grub2 search function to look for a filesystem
with the label "kernel". This works thanks to existing setup in
scripts/gen_image_generic.sh. Which sets the "kernel" label on
both the fat and ext4 filesystem variants.

Signed-off-by: Jax Jiang <jax.jiang.007@gmail.com>
Suggested-by: Alberto Bursi <bobafetthotmail@gmail.com> (MX100 WA)
(word wrapped, slightly rewritten commit message, removed MX100 WA)
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

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

@@ -65,16 +65,6 @@ platform_do_bootloader_upgrade() {
 			"/dev/$diskdev" \
 		&& touch /tmp/boot/boot/grub/upgraded
 
-        case "$(board_name)" in
-        cisco-mx100-hw)
-            # If the MX100 is booted UEFI AND the SATA HDD exists, we need to change
-            # grub's root= to hd1 for it to boot correctly, otherwise we can keep it hd0.
-            if [ -d /sys/firmware/efi ] && [ "$(ls -a /dev/sd[a-z] | wc -w)" -gt 1 ] ; then
-                sed -i "s|hd0,${parttable}1|hd1,${parttable}1|g" /tmp/boot/boot/grub/grub.cfg
-            fi
-            ;;
-        esac
-
 		umount /tmp/boot
 	fi
 }