kbuild: use relative path to include Makefile

The "MAKEFLAGS += --include-dir=$(srctree)" line in the top Makefile
allows us to do this.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>
diff --git a/scripts/Makefile.dtbinst b/scripts/Makefile.dtbinst
index 909ed7a..7cbff16 100644
--- a/scripts/Makefile.dtbinst
+++ b/scripts/Makefile.dtbinst
@@ -18,7 +18,7 @@
 
 include include/config/auto.conf
 include scripts/Kbuild.include
-include $(srctree)/$(obj)/Makefile
+include $(obj)/Makefile
 
 PHONY += __dtbs_install_prep
 __dtbs_install_prep: