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: NFrank Rowand <frank.rowand@sony.com>
Acked-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: NRob Herring <robh@kernel.org>

scripts/Makefile.lib

@@ -293,6 +293,8 @@ DTC_FLAGS += -Wnode_name_chars_strict \
 	-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=						\