grub: add rootwait in cmdline

In some configuration in SECO-B68 (using a USB drive or sata),
it can happen that boot devices do not show up. It needs 'rootwait'
to prevent panic.

Signed-off-by: Ettore Chimenti <ettore.chimenti@seco.com>

meta-oniro-core/recipes-bsp/grub/files/grub.cfg

@@ -7,7 +7,7 @@
 
 serial --unit=0 --speed=115200 --word=8 --parity=no --stop=1
 
-set CMDLINE="console=ttyS0,115200 panic=60 quiet"
+set CMDLINE="console=ttyS0,115200 panic=60 rootwait quiet"
 
 # Find the boot partition. We need to know where to load from and save the
 # environment to. The boot partition is saved into the ONIRO_GRUB_BOOT