acpi: Quieten IASL output when 'make -s' is used

IASL compiler does not provide a command line option to turn off
its non-warning message. To quieten the output when 'make -s',
redirect its output to /dev/null.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

scripts/Makefile.lib

@@ -326,7 +326,7 @@ $(obj)/%.S: $(src)/%.ttf
 quiet_cmd_acpi_c_asl= ASL     $<
 cmd_acpi_c_asl=         \
 	$(CPP) -x assembler-with-cpp -P $(UBOOTINCLUDE) -o $<.tmp $<; \
-	iasl -p $< -tc $<.tmp; \
+	iasl -p $< -tc $<.tmp $(if $(KBUILD_VERBOSE:1=), >/dev/null); \
 	mv $(patsubst %.asl,%.hex,$<) $@
 
 $(obj)/dsdt.c:    $(src)/dsdt.asl