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/spl/Makefile b/spl/Makefile
index b89a579..be5fd3b 100644
--- a/spl/Makefile
+++ b/spl/Makefile
@@ -166,7 +166,7 @@
 # on the fly.
 LDPPFLAGS += \
 	-include $(srctree)/include/u-boot/u-boot.lds.h \
-	-include $(OBJTREE)/include/config.h \
+	-include $(objtree)/include/config.h \
 	-DCPUDIR=$(CPUDIR) \
 	$(shell $(LD) --version | \
 	  sed -ne 's/GNU ld version \([0-9][0-9]*\)\.\([0-9][0-9]*\).*/-DLD_MAJOR=\1 -DLD_MINOR=\2/p')
@@ -197,9 +197,9 @@
 VAR_SIZE_PARAM =
 endif
 $(obj)/$(BOARD)-spl.bin: $(obj)/u-boot-spl.bin
-	$(if $(wildcard $(OBJTREE)/spl/board/samsung/$(BOARD)/tools/mk$(BOARD)spl),\
-	$(OBJTREE)/spl/board/samsung/$(BOARD)/tools/mk$(BOARD)spl,\
-	$(OBJTREE)/tools/mkexynosspl) $(VAR_SIZE_PARAM) $< $@
+	$(if $(wildcard $(objtree)/spl/board/samsung/$(BOARD)/tools/mk$(BOARD)spl),\
+	$(objtree)/spl/board/samsung/$(BOARD)/tools/mk$(BOARD)spl,\
+	$(objtree)/tools/mkexynosspl) $(VAR_SIZE_PARAM) $< $@
 endif
 
 quiet_cmd_objcopy = OBJCOPY $@