dtbo: Fix dtbo generation rules

Take the first prerequisite (dts overlay file) instead of standard
input.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

scripts/Makefile.lib

@@ -317,7 +317,7 @@ quiet_cmd_dtco = DTCO    $@
 # No generation of assembly file either
 # Modified for U-Boot
 cmd_dtco = mkdir -p $(dir ${dtc-tmp}) ; \
-	$(CPP) $(dtc_cpp_flags) -x assembler-with-cpp -o $(dtc-tmp) - ; \
+	$(CPP) $(dtc_cpp_flags) -x assembler-with-cpp -o $(dtc-tmp) $< ; \
 	$(DTC) -@ -O dtb -o $@ -b 0 \
 		-i $(dir $<) $(DTC_FLAGS) \
 		-d $(depfile).dtc.tmp $(dtc-tmp) ; \