mtd: ubi: Select RBTREE option from MTD_UBI Kconfig entry

Expose the RBTREE feature through Kconfig and select this option from the
MTD_UBI option.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Jagan Teki <jagan@openedev.com>
[Rebased on master]
Signed-off-by: Jagan Teki <jagan@openedev.com>
diff --git a/lib/Kconfig b/lib/Kconfig
index 5115d04..1048068 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -61,6 +61,9 @@
 	  size-constrained envrionments even this may be too big. Enable this
 	  option to reduce code size slightly at the cost of some speed.
 
+config RBTREE
+	bool
+
 source lib/dhry/Kconfig
 
 source lib/rsa/Kconfig