buildman: Implement an option to exclude boards from the build

Some boards are known to be broken and it is convenient to be able to
exclude them from the build.

Add an --exclude option to specific boards to exclude. This uses the
same matching rules as the normal 'include' arguments, and is a comma-
separated list of regular expressions.

Suggested-by: York Sun <yorksun@freescale.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/README b/tools/buildman/README
index 68465b4..b8c2bd6 100644
--- a/tools/buildman/README
+++ b/tools/buildman/README
@@ -114,6 +114,13 @@
 * 'freescale & arm sandbox'  All Freescale boards with ARM architecture,
                              plus sandbox
 
+You can also use -x to specifically exclude some boards. For example:
+
+ buildmand arm -x nvidia,freescale,.*ball$
+
+means to build all arm boards except nvidia, freescale and anything ending
+with 'ball'.
+
 It is convenient to use the -n option to see whaat will be built based on
 the subset given.