kbuild: rename TOPDIR to stctree

Prior to Kbuild, $(TOPDIR) or $(SRCTREE) was used for
pointing to the top of source directory.
(No difference between the two.)

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

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 8b29e97..e08a9a0 100644
--- a/nand_spl/board/sheldon/simpc8313/Makefile
+++ b/nand_spl/board/sheldon/simpc8313/Makefile
@@ -11,7 +11,7 @@
 
 nandobj	:= $(OBJTREE)/nand_spl/
 
-LDSCRIPT= $(TOPDIR)/nand_spl/board/$(BOARDDIR)/u-boot.lds
+LDSCRIPT= $(srctree)/nand_spl/board/$(BOARDDIR)/u-boot.lds
 LDFLAGS := -T $(nandobj)u-boot.lds -Ttext $(CONFIG_SYS_TEXT_BASE_SPL) \
 	   $(LDFLAGS) $(LDFLAGS_FINAL)
 asflags-y += -DCONFIG_NAND_SPL