add necessary code to generate mtd-symlinks for kernel-2.6

* symlinks are /dev/mtd/? -> /dev/mtd? and /dev/mtd/?ro -> /dev/mtd?ro
* needed at least for fon2100 to allow fwcf to find it's partition
* generally, this would be a mdev task, but since mdev sux having an
  additional script is the cleanest solution