This is a breaking change: applications which used diskio.h to
call ff_diskio_register, will now need to include diskio_impl.h.
Including diskio.h will include the original diskio.h header from
FatFS library.
Current implementation has drive numbers and paths hardcoded to support
only one FATFS drive. Arduino has it's own SPI driver to allow
compatibility and flexibility. With the MR it is possible to have up to
```_VOLUMES``` drives connected (SPI, SDMMC and others) at the same
time and accessed through VFS
