riscv: support Overlay Buildling

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>

scripts/Makefile.lib

@@ -381,6 +381,24 @@ endef
 $(obj)/%.dt.yaml: $(src)/%.dts $(DTC) $(DT_TMP_SCHEMA) FORCE
 	$(call if_changed_rule,dtc,yaml)
 
+quiet_cmd_dtco = DTCO    $@
+cmd_dtco = mkdir -p $(dir ${dtc-tmp}) ; \
+	$(CPP) $(dtc_cpp_flags) -x assembler-with-cpp -o $(dtc-tmp) $< ; \
+	$(DTC) -@ -H epapr -O dtb -o $@ -b 0 \
+		-i $(dir $<) $(DTC_FLAGS) \
+		-Wno-interrupts_property \
+		-Wno-label_is_string \
+		-Wno-reg_format \
+		-Wno-pci_device_bus_num \
+		-Wno-i2c_bus_reg \
+		-Wno-spi_bus_reg \
+		-Wno-avoid_default_addr_size \
+		-d $(depfile).dtc.tmp $(dtc-tmp) ; \
+	cat $(depfile).pre.tmp $(depfile).dtc.tmp > $(depfile)
+
+$(obj)/%.dtbo: $(src)/%.dts FORCE
+	$(call if_changed_dep,dtco)
+
 dtc-tmp = $(subst $(comma),_,$(dot-target).dts.tmp)
 
 # Bzip2