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/spl/Makefile b/spl/Makefile
index bb3d349..640db8b 100644
--- a/spl/Makefile
+++ b/spl/Makefile
@@ -44,7 +44,7 @@
   -include include/spl-autoconf.mk
 endif
 
-include $(TOPDIR)/config.mk
+include $(srctree)/config.mk
 
 # Enable garbage collection of un-used sections for SPL
 KBUILD_CFLAGS += -ffunction-sections -fdata-sections
@@ -149,13 +149,13 @@
 endif
 
 ifeq ($(wildcard $(LDSCRIPT)),)
-	LDSCRIPT := $(TOPDIR)/board/$(BOARDDIR)/u-boot-spl.lds
+	LDSCRIPT := $(srctree)/board/$(BOARDDIR)/u-boot-spl.lds
 endif
 ifeq ($(wildcard $(LDSCRIPT)),)
-	LDSCRIPT := $(TOPDIR)/$(CPUDIR)/u-boot-spl.lds
+	LDSCRIPT := $(srctree)/$(CPUDIR)/u-boot-spl.lds
 endif
 ifeq ($(wildcard $(LDSCRIPT)),)
-	LDSCRIPT := $(TOPDIR)/arch/$(ARCH)/cpu/u-boot-spl.lds
+	LDSCRIPT := $(srctree)/arch/$(ARCH)/cpu/u-boot-spl.lds
 endif
 ifeq ($(wildcard $(LDSCRIPT)),)
 $(error could not find linker script)
@@ -165,7 +165,7 @@
 # Pass the version down so we can handle backwards compatibility
 # on the fly.
 LDPPFLAGS += \
-	-include $(TOPDIR)/include/u-boot/u-boot.lds.h \
+	-include $(srctree)/include/u-boot/u-boot.lds.h \
 	-include $(OBJTREE)/include/config.h \
 	-DCPUDIR=$(CPUDIR) \
 	$(shell $(LD) --version | \