Fixed #560 Unable to upcast legacy variant

Refactored legacy variant to remove upcast functors.
Added upcast_ptr, is_base_of & 'not a base' error exception to legacy variant.
If ETL_USE_LEGACY_VARIANT is defined then legacy variant is in the etl namespace.
If ETL_USE_LEGACY_VARIANT is not defined then legacy variant is in the etl::legacy namespace.
Added non-member etl::send_message for etl::shared_message
Green Hills compiler compatibility