board/qemu/aarch64-sbsa: use PARTLABEL to specify root

Specifying /dev/sda2 is ambigious, and when booting my arm64 machine
this tries to mount the second partition of my ATA drive, instead of
the second partition of the USB-stick, simply because the ATA driver
is probed earlier than the USB controller driver.

To solve this problem, use PARTLABEL=root to specify the root filesystem.
This is similar to how it is currently done for board/aarch64-efi/grub.cfg
and board/qemu/aarch64-ebbr/grub.cfg.

Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/qemu/aarch64-sbsa/grub.cfg

@@ -2,5 +2,5 @@ set default="0"
 set timeout="5"
 
 menuentry "Buildroot" {
-	linux /Image root=/dev/sda2 rootwait console=ttyAMA0
+	linux /Image root=PARTLABEL=root rootwait console=ttyAMA0
 }