kbuild: fix # escaping in appending U-Boot own DT

The escape sequence '\#' does not work for the latest GNU Make from
the git tree.

Replace it with $(pound) as Linux did.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Author: masahir0y

scripts/Makefile.lib

@@ -308,7 +308,7 @@ quiet_cmd_dtc = DTC     $@
 # Modified for U-Boot
 # Bring in any U-Boot-specific include at the end of the file
 cmd_dtc = mkdir -p $(dir ${dtc-tmp}) ; \
-	(cat $<; $(if $(u_boot_dtsi),echo '\#include "$(u_boot_dtsi)"')) > $(pre-tmp); \
+	(cat $<; $(if $(u_boot_dtsi),echo '$(pound)include "$(u_boot_dtsi)"')) > $(pre-tmp); \
 	$(CPP) $(dtc_cpp_flags) -x assembler-with-cpp -o $(dtc-tmp) $(pre-tmp) ; \
 	$(DTC) -O dtb -o $@ -b 0 \
 		-i $(dir $<) $(DTC_FLAGS) \