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.
This change allows readonly FATFS to be mounted without wear levelling
support. This will provide the customers a simple way to mount FATFS images
generated on host and flashed onto the chip during factory provisioning.
Since NVS encryption is not supported yet and NVS entry size is limited,
the change will provide an easy alternative for securing the provisioning data
by just marking FATFS parition as encrypted.
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
