buildman: Fix repeating board list with -l

Ensure that we don't print duplicate board names when -l is used.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Albert Aribaud <albert.u.boot@aribaud.net>

tools/buildman/builder.py

@@ -918,7 +918,8 @@ class Builder:
             if self._list_error_boards:
                 names = []
                 for board in line_boards[line]:
-                    names.append(board.target)
+                    if not board.target in names:
+                        names.append(board.target)
                 names_str = '(%s) ' % ','.join(names)
             else:
                 names_str = ''