UBI: Enable re-initializing of the "ubi part" command

With this patch now, the user can call "ubi part" multiple times to
re-connect the UBI device to another MTD partition.

Signed-off-by: Stefan Roese <sr@denx.de>

include/ubi_uboot.h

@@ -211,6 +211,7 @@ static inline long IS_ERR(const void *ptr)
 /* functions */
 extern int ubi_mtd_param_parse(const char *val, struct kernel_param *kp);
 extern int ubi_init(void);
+extern void ubi_exit(void);
 
 extern struct ubi_device *ubi_devices[];