kbuild: fix warning "overriding recipe for target 'Makefile'"

If you do "make Makefile" with GNU Make 3.x, the following warning
is displayed:

  $ make Makefile
  Makefile:165: warning: overriding recipe for target 'Makefile'
  Makefile:51: warning: ignoring old recipe for target 'Makefile'
  make[1]: Nothing to be done for 'Makefile'.
  make: Nothing to be done for 'Makefile'.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -161,7 +161,7 @@ ifeq ($(need-sub-make),1)
 
 PHONY += $(MAKECMDGOALS) sub-make
 
-$(filter-out _all sub-make $(CURDIR)/Makefile, $(MAKECMDGOALS)) _all: sub-make
+$(filter-out _all sub-make $(lastword $(MAKEFILE_LIST)), $(MAKECMDGOALS)) _all: sub-make
 	@:
 
 # Invoke a second make in the output directory, passing relevant variables