buildman: Write out the build command used

It is sometimes useful to see the exact 'make' command used by buildman
for a commit. Add an output file for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/buildman.rst b/tools/buildman/buildman.rst
index 9a2d913..11c7214 100644
--- a/tools/buildman/buildman.rst
+++ b/tools/buildman/buildman.rst
@@ -1300,6 +1300,14 @@
 since it may be dropped altogether in future.
 
 
+Checking the command
+--------------------
+
+Buildman writes out the toolchain information to a `toolchain` file within the
+output directory. It also writes the commands used to build U-Boot in an
+`out-cmd` file. You can check these if you suspect something strange is
+happening.
+
 TODO
 ----