This directory-scoped varible is now used by config_option,
config_choice and config_string to set a created CMake cache variable as
advanced or not. An advanced variable is hidden by default in the CMake
configuration editors. Setting SEL4_CONFIG_DEFAULT_ADVANCED to ON will
cause variables to be advanced and not show up in the cache. Projects
can set this to limit the amount of options presented in the
config editor. Any cache variable can have this overridden by calling
mark_as_advanced(CLEAR config_name)
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