fdt: avoid bad MAKEALL status

Current versions of  dtc  always print a message like

	DTC: dts->dtb  on file "dt.dtb.tmp"

which cannot even be suppressed with "-qqq".  To avoid incorrect
MAKEALL status, we manually filter out this message.  This is a bit
complicated, as we have to make sure to set a correct return code.

Also, get rid of the temp file: dtc accepts "-" for stdin.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>

dts/Makefile

@@ -46,9 +46,13 @@ all:	$(obj).depend $(LIB)
 DT_BIN	:= $(obj)dt.dtb
 
 $(DT_BIN): $(TOPDIR)/board/$(VENDOR)/dts/$(DEVICE_TREE).dts
-	cat $< | $(CPP) -P $(DTS_CPPFLAGS) - >$@.tmp
-	$(DTC) -R 4 -p 0x1000 -O dtb -o ${DT_BIN} $@.tmp
-	rm $@.tmp
+	rc=$$( \
+		cat $< | $(CPP) -P $(DTS_CPPFLAGS) - | \
+		{ { $(DTC) -R 4 -p 0x1000 -O dtb -o ${DT_BIN} - 2>&1 ; \
+		    echo $$? >&3 ; } | \
+		  grep -v '^DTC: dts->dtb  on file' ; \
+	        } 3>&1 ) ; \
+	exit $$rc
 
 process_lds = \
 	$(1) | sed -r -n 's/^OUTPUT_$(2)[ ("]*([^")]*).*/\1/p'