buildman: Add support for building with clang

Add a -O option which allows building with clang.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/README b/tools/buildman/README
index d688b7c..56a99c7 100644
--- a/tools/buildman/README
+++ b/tools/buildman/README
@@ -1046,6 +1046,16 @@
 
 The -U option uses the u-boot.env files which are produced by a build.
 
+
+Building with clang
+===================
+
+To build with clang (sandbox only), use the -O option to override the
+toolchain. For example:
+
+   buildman -O clang-7 --board sandbox
+
+
 Other options
 =============