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>
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 66e5015..704d644 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -317,7 +317,7 @@
 # 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) ; \