kbuild: merge scripts/mkmakefile to top Makefile

scripts/mkmakefile is simple enough to be merged in the Makefile.

Use $(call cmd,...) to show the log instead of doing it in the
shell script.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -544,14 +544,21 @@ scripts_basic:
 	$(Q)rm -f .tmp_quiet_recordmcount
 
 PHONY += outputmakefile
+ifdef building_out_of_srctree
 # Before starting out-of-tree build, make sure the source tree is clean.
 # outputmakefile generates a Makefile in the output directory, if using a
 # separate output directory. This allows convenient use of make in the
 # output directory.
 # At the same time when output Makefile generated, generate .gitignore to
 # ignore whole output directory
+
+quiet_cmd_makefile = GEN     Makefile
+      cmd_makefile = { \
+	echo "\# Automatically generated by $(srctree)/Makefile: don't edit"; \
+	echo "include $(srctree)/Makefile"; \
+	} > Makefile
+
 outputmakefile:
-ifdef building_out_of_srctree
 	$(Q)if [ -f $(srctree)/.config -o \
 		 -d $(srctree)/include/config -o \
 		 -d $(srctree)/arch/$(SRCARCH)/include/generated ]; then \
@@ -562,7 +569,7 @@ ifdef building_out_of_srctree
 		false; \
 	fi
 	$(Q)ln -fsn $(srctree) source
-	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/mkmakefile $(srctree)
+	$(call cmd,makefile)
 	$(Q)test -e .gitignore || \
 	{ echo "# this is build directory, ignore it"; echo "*"; } > .gitignore
 endif