fatfs: Add raw emuMMC support for USER partition

bdk/libs/fatfs/diskio.h

@@ -27,7 +27,8 @@ typedef enum {
 	DRIVE_SD   = 0,
 	DRIVE_RAM  = 1,
 	DRIVE_EMMC = 2,
-	DRIVE_BIS  = 3
+	DRIVE_BIS  = 3,
+	DRIVE_EMU  = 4
 } DDRIVE;