The `mtd_default` module defines `MTD_NUMOF` if not existing based on the `MTD_*` defines which are usually set to the corresponding MTD device pointer variables `mtd*`. However, these MTD device pointer variables are not always made known by external variable declarations. An example are SD Card Interfaces which are defined via the `mtd_sdcard_default` module. As a consequence, an application that uses `mtd_default` has still to be modified. Therefore, `mtd_default` also declares up to 6 `mtd*` MTD device pointer variables.
