binman: Add a way to enable debugging from the build

When the build fails due to something wrong in binman it is sometimes
useful to get a full backtrace showing the location of the failure. Add
a BINMAN_DEBUG environment variable to support this along with some
documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/README b/tools/binman/README
index b64dedf..04ed2b79 100644
--- a/tools/binman/README
+++ b/tools/binman/README
@@ -723,6 +723,12 @@
 the DTC environment variable. This can be useful when the system dtc is too
 old.
 
+To enable a full backtrace and other debugging features in binman, pass
+BINMAN_DEBUG=1 to your build:
+
+   make sandbox_defconfig
+   make BINMAN_DEBUG=1
+
 
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