dts/Makefile: simplify dtc invocation

The invocation of dtc is significantly more complex that it could be,
in order to work around an issue on old versions of dtc, which print
a message to stdout every time they run.

Remove this workaround, on the assumption that people have or will
upgrade to a newer version of dtc. This simplifies the build rule
significantly.

Related, split the invocation of cpp and dtc into separate commands
rather than a pipeline, so that if either fail, it is detected. This has
the nice benefit of saving off the result of the pre-processing step,
allowing it to be easily inspected.

Assuming a new enough dtc (which an earlier patch enforces), dtc will
parse #line directives in its input file, and generate correct file and
line numbers in error messages, even though cpp is unconditionally
applied to its input file.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>

.gitignore

@@ -16,6 +16,7 @@
 *.patch
 *.bin
 *.cfgtmp
+*.dts.tmp
 
 # Build tree
 /build-*

dts/Makefile

@@ -35,13 +35,8 @@ all:	$(obj).depend $(LIB)
 DT_BIN	:= $(obj)dt.dtb
 
 $(DT_BIN): $(TOPDIR)/board/$(VENDOR)/dts/$(DEVICE_TREE).dts
-	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 1>&2 ) ; \
-	exit $$rc
+	$(CPP) -P $(DTS_CPPFLAGS) $< -o $(DT_BIN).dts.tmp
+	$(DTC) -R 4 -p 0x1000 -O dtb -o ${DT_BIN} $(DT_BIN).dts.tmp
 
 process_lds = \
 	$(1) | sed -r -n 's/^OUTPUT_$(2)[ ("]*([^")]*).*/\1/p'