How to configure an additional field for a bundle that is created by a custom module
You have to add the field via the UI.
Then you can export the yml config files required by the field by 


Using drush cex sync
Navigating to admin/config/development/configuration/single/export and export the changed/new files on admin/config/development/configuration manually.


Copy the new/changed files to your config/install folder and you are good to go