Add Red Black Tree support

Now it's used at UBI module. Of course other modules can use it.
If you want to use it, please define CONFIG_RBTREE

Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
diff --git a/lib_generic/Makefile b/lib_generic/Makefile
index 7df5a2c..d62c39b 100644
--- a/lib_generic/Makefile
+++ b/lib_generic/Makefile
@@ -45,6 +45,7 @@
 COBJS-y	+= strmhz.o
 COBJS-y += vsprintf.o
 COBJS-y += zlib.o
+COBJS-$(CONFIG_RBTREE)	+= rbtree.o
 
 COBJS	:= $(COBJS-y)
 SRCS	:= $(COBJS:.o=.c)