kbuild: finally remove the obsolete variable $TOPDIR

TOPDIR is obsolete, it can be finally removed now.

Signed-off-by: WANG Cong <amwang@redhat.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

diff --git a/Makefile b/Makefile
index 8fb9bfc..b4c7ef5 100644
--- a/Makefile
+++ b/Makefile
@@ -140,15 +140,13 @@
 endif
 
 srctree		:= $(if $(KBUILD_SRC),$(KBUILD_SRC),$(CURDIR))
-TOPDIR		:= $(srctree)
-# FIXME - TOPDIR is obsolete, use srctree/objtree
 objtree		:= $(CURDIR)
 src		:= $(srctree)
 obj		:= $(objtree)
 
 VPATH		:= $(srctree)$(if $(KBUILD_EXTMOD),:$(KBUILD_EXTMOD))
 
-export srctree objtree VPATH TOPDIR
+export srctree objtree VPATH
 
 
 # SUBARCH tells the usermode build what the underlying arch is.  That is set