kbuild: prefix Makefile.dtbinst path with $(srctree) unconditionally

$(srctree) always points to the top of the source tree whether
KBUILD_SRC is set or not.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/Kbuild.include

@@ -193,7 +193,7 @@ modbuiltin := -f $(srctree)/scripts/Makefile.modbuiltin obj
 # Shorthand for $(Q)$(MAKE) -f scripts/Makefile.dtbinst obj=
 # Usage:
 # $(Q)$(MAKE) $(dtbinst)=dir
-dtbinst := -f $(if $(KBUILD_SRC),$(srctree)/)scripts/Makefile.dtbinst obj
+dtbinst := -f $(srctree)/scripts/Makefile.dtbinst obj
 
 ###
 # Shorthand for $(Q)$(MAKE) -f scripts/Makefile.clean obj=