FATFS enable support for multiple drives

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

components/fatfs/src/diskio.h

@@ -76,6 +76,13 @@ void ff_diskio_register(BYTE pdrv, const ff_diskio_impl_t* discio_impl);
  */
 void ff_diskio_register_sdmmc(BYTE pdrv, sdmmc_card_t* card);
 
+/**
+ * Get next available drive number
+ *
+ * @return 0xFF on fail, else the drive number
+ */
+BYTE ff_disk_getpdrv();
+
 /* Disk Status Bits (DSTATUS) */
 
 #define STA_NOINIT		0x01	/* Drive not initialized */