Windows Check override setting is not working

The new windows warning feature have some problems when trying to avoid it using the drushrc.php $options['check_os'] = FALSE;

The function drush_environment_check_os() is called in the drush bootstrap drush phase validation and on this phase (well pre-validation phase) there is not yet any drush_load_config('drush') so the drush_get_option('check-os', TRUE) doesn't return anything because there are not yet any read options from the config files.
The options are adding the drush_load_config in the check os function or move this check to the next bootstrap root phase validation or move this check to the bootstrap drush function after the config files have been read.

I considered that this check should be done in the drush bootstrap phase so this patch moves the check function to the function _drush_bootstrap_drush() instead than in the function _drush_bootstrap_drush_validate().