kbuild: rename OBJTREE to objtree

Prior to Kbuild, $(OBJTREE) was used for pointing to the
top of build directory with absolute path.

In Kbuild style, $(objtree) is used instead.
This commit renames OBJTREE to objtree and delete the
defition of OBJTREE.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/nand_spl/board/sheldon/simpc8313/Makefile b/nand_spl/board/sheldon/simpc8313/Makefile
index 664301c..657f65f 100644
--- a/nand_spl/board/sheldon/simpc8313/Makefile
+++ b/nand_spl/board/sheldon/simpc8313/Makefile
@@ -9,7 +9,7 @@
 
 include $(srctree)/$(src)/config.mk
 
-nandobj	:= $(OBJTREE)/nand_spl/
+nandobj	:= $(objtree)/nand_spl/
 
 LDSCRIPT= $(srctree)/nand_spl/board/$(BOARDDIR)/u-boot.lds
 LDFLAGS := -T $(nandobj)u-boot.lds -Ttext $(CONFIG_SYS_TEXT_BASE_SPL) \