buildman: Make sure that -o is given with -w

It is a bad idea to use the default output directory ('..') with -w since
it does a build in that directory and writes various files these.

Require that -o is given to avoid this problem.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/README b/tools/buildman/README
index f3a0dc7..b2f983c 100644
--- a/tools/buildman/README
+++ b/tools/buildman/README
@@ -1091,7 +1091,8 @@
 
    buildman -o /tmp/build --board sandbox -w
 
-This will write the full build into /tmp/build including object files.
+This will write the full build into /tmp/build including object files. You must
+specify the output directory with -o when using -w.
 
 
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