installer: copy 1M zeroes to the target device before running fdisk

otherwise grub-install chokes, even after the new partition table has been created

scripts/installer/set-disk-partitions

@@ -28,11 +28,11 @@ if [ "$PARTITION_COUNT" -gt "1" ]; then
             exit 1
         fi
     done
-
-    dd if=/dev/zero of=${DEVICE} bs=512 count=1
-    partprobe ${DEVICE}
 fi
 
+dd if=/dev/zero of=${DEVICE} bs=512 count=2048
+partprobe ${DEVICE}
+
 fdisk ${DEVICE} <<EOF
 n
 p