kbuild: check clean srctree even earlier

Move the outputmakefile target to the leftmost in the prerequisite list
so that this is checked first. GNU Make processes the prerequisites
left to right.

GNU Make will keep to stick to this behavior, and it seems even
POSIX standard, according to this:
https://lists.gnu.org/archive/html/bug-make/2019-08/msg00030.html

The POSIX standard of make is available here:
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/make.html

Of course, when the parallel option -j given, other targets will be run
simultaneously but it is nice to show the error as early as possible.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -578,10 +578,10 @@ ifdef config-build
 include arch/$(SRCARCH)/Makefile
 export KBUILD_DEFCONFIG KBUILD_KCONFIG CC_VERSION_TEXT
 
-config: scripts_basic outputmakefile FORCE
+config: outputmakefile scripts_basic FORCE
 	$(Q)$(MAKE) $(build)=scripts/kconfig $@
 
-%config: scripts_basic outputmakefile FORCE
+%config: outputmakefile scripts_basic FORCE
 	$(Q)$(MAKE) $(build)=scripts/kconfig $@
 
 else #!config-build
@@ -1120,7 +1120,7 @@ scripts: scripts_basic scripts_dtc
 
 PHONY += prepare archprepare
 
-archprepare: archheaders archscripts scripts include/config/kernel.release outputmakefile \
+archprepare: outputmakefile archheaders archscripts scripts include/config/kernel.release \
 	asm-generic $(version_h) $(autoksyms_h) include/generated/utsrelease.h
 
 prepare0: archprepare