fdt: Show the preprocessed .dts file on error

When device-tree compilation fails it is sometimes tricky to see which
line is broken, since the input file to dtc is a pre-processed version
of the device tree.

Add a line that points to the file that needs to be checked:

When the error is in the main .dts file, output is something like this:

   output: 'Error: arch/x86/dts/.chromebook_coral.dtb.pre.tmp:478.46-47
	syntax error
   FATAL ERROR: Unable to parse input tree

but in fact looking at that file shows nothing useful:

   PAD_CFG_NF_IOSSTATE_IOSTERM(GPIO_157, UP_20K, DEEP, NF1, HIZCRX1, DISPUPD)

Instead we need to look at the preprocessed file, which shows:

   163 ((1U << 30) | (1 << 10)) ((0xb << 10) | PAD_CFG1_IOSSTATE_HIZCRX1)

Here it is clear that PAD_CFG1_IOSSTATE_HIZCRX1 is not defined and so is
not being resolved by the preprocessor.

This commit adds an additional useful message:

   Check arch/x86/dts/.chromebook_coral.dtb.dts.tmp for errors

Note that if the error is reported in an included file, such as
u-boot.dtsi then the output is the following:

   Error: arch/x86/dts/u-boot.dtsi:137.14-15 syntax error
   FATAL ERROR: Unable to parse input tree

But again, if the error is due to a preprocessor failure, like this:

   filename = CONFIG_IFW_INPUT_FILE;

then you can't tell what the problem is by looking at the source. All you
see is the original code:

	intel-ifwi {
		filename = CONFIG_IFW_INPUT_FILE;
		...
		};
	};
	intel-fsp-m {
		filename = CONFIG_FSP_FILE_M;
	};

Everything looks fine. But looking at the output of the preprocessor:

 intel-ifwi {
  filename = CONFIG_IFW_INPUT_FILE;
  ...
 };
 intel-fsp-m {
  filename = "fsp_m.bin";
 };

This shows that the filename (normally "fitimage.bin") has not been
inserted the preprocess, leading to the realisation that the value should
be CONFIG_IFWI_INPUT_FILE.

If the above does not make sense, I encourage people to try introducing
errors in the device tree preprocessed values.

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

scripts/Makefile.lib

@@ -300,7 +300,9 @@ cmd_dtc = mkdir -p $(dir ${dtc-tmp}) ; \
 	$(CPP) $(dtc_cpp_flags) -x assembler-with-cpp -o $(dtc-tmp) $(pre-tmp) ; \
 	$(DTC) -O dtb -o $@ -b 0 \
 		-i $(dir $<) $(DTC_FLAGS) \
-		-d $(depfile).dtc.tmp $(dtc-tmp) ; \
+		-d $(depfile).dtc.tmp $(dtc-tmp) || \
+		(echo "Check $(shell pwd)/$(pre-tmp) for errors" && false) \
+		; \
 	cat $(depfile).pre.tmp $(depfile).dtc.tmp > $(depfile) ; \
 	sed -i "s:$(pre-tmp):$(<):" $(depfile)