If a certain image type can be created both by applying conversion commands to a base type as well as via a specialized command (like a fictitious IMAGE_CMD_ext4.xz, or some dedicated task, as indicated by IMAGE_TYPES_MASKED = ext4.xz), then it is better to start with the specialized command or task. That way developers can choose between using the builtin conversion support or re-implementing certain types, for example more efficiently without intermediate files. Out-dated patch is here: https://github.com/pohly/ostro-os/commit/be80d9330cccb496653d5c08b218b6e50e05b729