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/config.mk b/config.mk
index 4657577..9bf4eda 100644
--- a/config.mk
+++ b/config.mk
@@ -29,11 +29,11 @@
 CPUDIR=arch/$(ARCH)/cpu
 endif
 
-sinclude $(TOPDIR)/arch/$(ARCH)/config.mk	# include architecture dependend rules
-sinclude $(TOPDIR)/$(CPUDIR)/config.mk		# include  CPU	specific rules
+sinclude $(srctree)/arch/$(ARCH)/config.mk	# include architecture dependend rules
+sinclude $(srctree)/$(CPUDIR)/config.mk		# include  CPU	specific rules
 
 ifdef	SOC
-sinclude $(TOPDIR)/$(CPUDIR)/$(SOC)/config.mk	# include  SoC	specific rules
+sinclude $(srctree)/$(CPUDIR)/$(SOC)/config.mk	# include  SoC	specific rules
 endif
 ifneq ($(BOARD),)
 ifdef	VENDOR
@@ -43,7 +43,7 @@
 endif
 endif
 ifdef	BOARD
-sinclude $(TOPDIR)/board/$(BOARDDIR)/config.mk	# include board specific rules
+sinclude $(srctree)/board/$(BOARDDIR)/config.mk	# include board specific rules
 endif
 
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