modules: Add placeholder for CONFIG_RUST

In order to be able to ifdef based on `CONFIG_RUST`, the config must be
defined in the main repo (because the rust module is optional).  Add
this as a placeholder.  If the module is not availabe, the symbol will
be present, and always disabled.  The module will provide a full
definition for the Kconfig.

Signed-off-by: David Brown <david.brown@linaro.org>