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

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>