Boards that are shipped with a DFU bootloader define the `dfu-util` configuration in their `Makefile.include`. However, when `riotboot_dfu` is used as the DFU bootloader, the board's `dfu-util` configuration must be overridden by the configuration as required by `riotboot_dfu` to use it to flash applications. Therefore, all `dfu-util` configurations are defined as overridable in the board's `Makefile.include` and the configuration as required by `riotboot_dfu` is included before the board's `Makefile.include`.
