kbuild: use absolute path in the generated wrapper Makefile

Keep the consistent behavior when this Makefile is invoked from another
directory.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

Author: masahir0y

Makefile

@@ -644,8 +644,9 @@ ifdef building_out_of_srctree
 
 quiet_cmd_makefile = GEN     Makefile
       cmd_makefile = { \
-	echo "\# Automatically generated by $(srctree)/Makefile: don't edit"; \
-	echo "include $(srctree)/Makefile"; \
+	echo "\# Automatically generated by $(abs_srctree)/Makefile: don't edit"; \
+	echo "export KBUILD_OUTPUT = $(CURDIR)"; \
+	echo "include $(abs_srctree)/Makefile"; \
 	} > Makefile
 
 outputmakefile: