of: per-file dtc compiler flags

The dtc compiler version that adds initial support was available
in 4.11-rc1.  Add the ability to set an additional dtc compiler
flag is needed by overlays.

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Rob Herring <robh@kernel.org>
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 8066b618..01c7740 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -293,6 +293,8 @@
 	-Wproperty_name_chars_strict
 endif
 
+DTC_FLAGS += $(DTC_FLAGS_$(basetarget))
+
 # Generate an assembly file to wrap the output of the device tree compiler
 quiet_cmd_dt_S_dtb= DTB     $@
 cmd_dt_S_dtb=						\