kbuild: reduce output spam when building out of tree

The execution of $(call cmd,makefile) will print 'GEN Makefile' on each
build, even if the Makefile is not effectively changed.

Use a filechk command instead, so a message is only printed on changes.

The Makefile is now created even if the build is aborted due to an
unclean working tree. That should not make a difference in practice.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20260305-kbuild-makefile-spam-v1-1-910f6cf218a1@linutronix.de
Signed-off-by: Nicolas Schier <nsc@kernel.org>

Makefile

@@ -675,14 +675,19 @@ print_env_for_makefile = \
 	echo "export KBUILD_OUTPUT = $(CURDIR)"
 endif
 
-quiet_cmd_makefile = GEN     Makefile
-      cmd_makefile = { \
+filechk_makefile = { \
 	echo "\# Automatically generated by $(abs_srctree)/Makefile: don't edit"; \
 	$(print_env_for_makefile); \
 	echo "include $(abs_srctree)/Makefile"; \
-	} > Makefile
+	}
 
-outputmakefile:
+$(objtree)/Makefile: FORCE
+	$(call filechk,makefile)
+
+# Prevent $(srcroot)/Makefile from inhibiting the rule to run.
+PHONY += $(objtree)/Makefile
+
+outputmakefile: $(objtree)/Makefile
 ifeq ($(KBUILD_EXTMOD),)
 	@if [ -f $(srctree)/.config -o \
 		 -d $(srctree)/include/config -o \
@@ -703,7 +708,6 @@ else
 	fi
 endif
 	$(Q)ln -fsn $(srcroot) source
-	$(call cmd,makefile)
 	$(Q)test -e .gitignore || \
 	{ echo "# this is build directory, ignore it"; echo "*"; } > .gitignore
 endif