arm/qemu-efi-disk: add rootwait to bootargs

Adding "rootwait" to bootargs for uniformity with the other wic files,
and this _could_ resolve Yocto Bugzilla Bug 15562 (as the intermittent
inability to find the root disk could be because of a race between
needing the disk and it not being mounted yet).

Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm/wic/qemu-efi-disk.wks.in

@@ -8,4 +8,4 @@ part / --source rootfs --fstype=ext4 --label root --align 1024 --use-uuid
 
 part swap --size 44 --label swap --fstype=swap --use-uuid
 
-bootloader --ptable gpt --timeout=5 --append="rootfstype=ext4 ip=dhcp"
+bootloader --ptable gpt --timeout=5 --append="rootwait rootfstype=ext4 ip=dhcp"