mk_mmc.sh: we need to nuke the partition labels, otherwise mke2fs 1.42.10 will ask for permission

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>

mk_mmc.sh

@@ -1075,7 +1075,9 @@ unmount_all_drive_partitions () {
 	done
 
 	echo "Zeroing out Partition Table"
-	dd if=/dev/zero of=${media} bs=1M count=16 || drive_error_ro
+	dd if=/dev/zero of=${media} bs=1M count=108 || drive_error_ro
+	sync
+	dd if=${media} of=/dev/null bs=1M count=108
 	sync
 }