buildman: Add an option to ignore migration warnings

These are becoming more common now. They cause boards to show warnings
which can be mistaking for compiler warnings.

Add a buildman option to ignore them. This option works only with the
summary option (-s). It does not affect the build process.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/cmdline.py b/tools/buildman/cmdline.py
index 8510c07..1377b9d 100644
--- a/tools/buildman/cmdline.py
+++ b/tools/buildman/cmdline.py
@@ -116,7 +116,10 @@
           help='Specify a list of boards to exclude, separated by comma')
     parser.add_option('-y', '--filter-dtb-warnings', action='store_true',
           default=False,
-           help='Filter out device-tree-compiler warnings from output')
+          help='Filter out device-tree-compiler warnings from output')
+    parser.add_option('-Y', '--filter-migration-warnings', action='store_true',
+          default=False,
+          help='Filter out migration warnings from output')
 
     parser.usage += """ [list of target/arch/cpu/board/vendor/soc to build]