kbuild: add this-makefile as a shorthand for $(lastword $(MAKEFILE_LIST))

Make it clearer, and self-documenting.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -157,12 +157,14 @@ MAKEFLAGS += --include-dir=$(abs_srctree)
 need-sub-make := 1
 endif
 
+this-makefile := $(lastword $(MAKEFILE_LIST))
+
 ifneq ($(filter 3.%,$(MAKE_VERSION)),)
 # 'MAKEFLAGS += -rR' does not immediately become effective for GNU Make 3.x
 # We need to invoke sub-make to avoid implicit rules in the top Makefile.
 need-sub-make := 1
 # Cancel implicit rules for this Makefile.
-$(lastword $(MAKEFILE_LIST)): ;
+$(this-makefile): ;
 endif
 
 export abs_srctree abs_objtree
@@ -172,7 +174,7 @@ ifeq ($(need-sub-make),1)
 
 PHONY += $(MAKECMDGOALS) sub-make
 
-$(filter-out _all sub-make $(lastword $(MAKEFILE_LIST)), $(MAKECMDGOALS)) _all: sub-make
+$(filter-out _all sub-make $(this-makefile), $(MAKECMDGOALS)) _all: sub-make
 	@:
 
 # Invoke a second make in the output directory, passing relevant variables