buildman: Separate out display of warnings and errors

Some boards unfortunately build with warnings and it is useful to be able
to easily distinguish the warnings from the errors.

Use a simple pattern match to categorise gcc output into warnings and
errors, and display each separately. New warnings are shown in magenta (with
a w+ prefix) and fixed warnings are shown in yellow with a w- prefix.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/README b/tools/buildman/README
index fbc8449..8ba19ec 100644
--- a/tools/buildman/README
+++ b/tools/buildman/README
@@ -445,6 +445,9 @@
 once. This makes the output as concise as possible. To see which boards have
 each error, use -l.
 
+Buildman tries to distinguish warnings from errors, and shows warning lines
+separately with a 'w' prefix.
+
 The full build output in this case is available in:
 
 ../lcd9b/12_of_18_gd92aff7_lcd--Add-support-for/lubbock/