Hello Lixianghan,
One alternative would be using the MCU Boot Utility (which you may download from the repository below), which does support the elf format directly.
https://github.com/JayHeng/NXP-MCUBootUtility
The tool has a GUI which is very intuitive. You may find more details on the Readme.md (main page of the repository when accessing from a web browser)
I hope that this information helps!
Regards,
Gustavo
