Avoid non-portable sed construct

Using \n in a substitution is a GNU extension.  Use the 'G" command instead
to insert the desired line.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>

scripts/Makefile.lib

@@ -311,7 +311,7 @@ quiet_cmd_dtc = DTC     $@
 # Bring in any U-Boot-specific include after the '/dts-v1/;' header
 cmd_dtc = mkdir -p $(dir ${dtc-tmp}) ; \
 	cat $< $(if $(u_boot_dtsi),\
-		| sed 's%^/ {$$%\#include \"$(u_boot_dtsi)\"\n&%')  | \
+		| sed '/^\/ {$$/{x;s%$$%\#include \"$(u_boot_dtsi)\"%;G;}') | \
 		$(CPP) $(dtc_cpp_flags) -x assembler-with-cpp -o $(dtc-tmp) - ; \
 	$(DTC) -O dtb -o $@ -b 0 \
 		-i $(dir $<) $(DTC_FLAGS) \