binman: Add better Makefile debugging

There is a debugging option in the Makefile to allow people to figure out
which u-boot.dtsi files are used in the build. But is it not easy to use
since it only shows files it finds, not those it is looking for. Update it
and update the mention of it to the docs.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/README b/tools/binman/README
index cb47e73..63f9722 100644
--- a/tools/binman/README
+++ b/tools/binman/README
@@ -446,7 +446,8 @@
 the 'warning' line in scripts/Makefile.lib to see what it has found:
 
    # Uncomment for debugging
-   # $(warning binman_dtsi_options: $(binman_dtsi_options))
+   # This shows all the files that were considered and the one that we chose.
+   # u_boot_dtsi_options_debug = $(u_boot_dtsi_options_raw)
 
 
 Code coverage