kbuild: move scripts/mod/ build to modules_prepare

Do $(build)=scripts/mod after $(build)=. so that
scripts/mod/devicetable-offsets.c can use headers generated by ./Kbuild.

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

Makefile

@@ -1191,7 +1191,6 @@ archprepare: outputmakefile archheaders archscripts scripts include/config/kerne
 	include/generated/autoconf.h remove-stale-files
 
 prepare0: archprepare
-	$(Q)$(MAKE) $(build)=scripts/mod
 	$(Q)$(MAKE) $(build)=.
 
 # All the preparing..
@@ -1438,6 +1437,7 @@ targets += modules.order
 PHONY += modules_prepare
 modules_prepare: prepare
 	$(Q)$(MAKE) $(build)=scripts scripts/module.lds
+	$(Q)$(MAKE) $(build)=scripts/mod
 
 export modules_sign_only :=