It's been several years since I've done this. However, I don't believe mkelfimage is needed.
You need to build a kernel image that contains the kernel, initramfs, and command line args. If my memory is correct, this is now possible with the standard kernel build process and it creates and ELF file. Since cbfstool can parse and ELF file, you should be able to use the standard coreboot flow.
I'm going to try to get this working anyway in a week or so so I'm interested to hear how it goes for you.