kbuild: reuse this-makefile to define abs_srctree

Move this-makefile up, and reuse it to define abs_srctree.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -145,7 +145,8 @@ else
 need-sub-make := 1
 endif
 
-abs_srctree := $(realpath $(dir $(lastword $(MAKEFILE_LIST))))
+this-makefile := $(lastword $(MAKEFILE_LIST))
+abs_srctree := $(realpath $(dir $(this-makefile)))
 
 ifneq ($(words $(subst :, ,$(abs_srctree))), 1)
 $(error source directory cannot contain spaces or colons)
@@ -160,8 +161,6 @@ MAKEFLAGS += --include-dir=$(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.