lib/oofatfs/ffconf.h: Allow to configure FS_EXFAT option.

Using MICROPY_FATFS_EXFAT.  Enabling this has licensing implications;
see https://www.microsoft.com/en-us/legal/intellectualproperty/mtl/exfat-licensing.aspx

lib/oofatfs/ffconf.h

@@ -268,7 +268,11 @@
 /  buffer in the file system object (FATFS) is used for the file data transfer. */
 
 
+#ifdef MICROPY_FATFS_EXFAT
+#define _FS_EXFAT   (MICROPY_FATFS_EXFAT)
+#else
 #define _FS_EXFAT   0
+#endif
 /* This option switches support of exFAT file system. (0:Disable or 1:Enable)
 /  When enable exFAT, also LFN needs to be enabled. (_USE_LFN >= 1)
 /  Note that enabling exFAT discards C89 compatibility. */