Change DISTORTOS_ACTIVE_CONFIGURATION_NAMES to a cache variable
Change DISTORTOS_ACTIVE_CONFIGURATION_NAMES to a cache variable

This variable is unset on first call to distortosSetConfiguration().
This way it may be used to keep the list of configuration variables from
previous CMake run.
DISTORTEC/distortos,DISTORTEC/distortos,DISTORTEC/distortos,DISTORTEC/distortos