boot/grub2: add rootwait to default config

It doesn't hurt, and is useful for removable boot media like a pendrive
that may depend on usb enumeration and isn't available immediately.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/grub2/grub.cfg

@@ -2,5 +2,5 @@ set default="0"
 set timeout="5"
 
 menuentry "Buildroot" {
-	linux /boot/bzImage root=/dev/sda1 console=tty1
+	linux /boot/bzImage root=/dev/sda1 rootwait console=tty1
 }