bulidman: Add support for a simple build

It is useful to run a simple build and put all the output in a single
directory. Add a -w option to support this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/README b/tools/buildman/README
index c1ac0d0..abbbbea 100644
--- a/tools/buildman/README
+++ b/tools/buildman/README
@@ -1056,6 +1056,17 @@
    buildman -O clang-7 --board sandbox
 
 
+Doing a simple build
+====================
+
+In some cases you just want to build a single board and get the full output, use
+the -w option, for example:
+
+   buildman -o /tmp/build --board sandbox -w
+
+This will write the full build into /tmp/build including object files.
+
+
 Other options
 =============