Last time we did this, at Sandia last summer, we just used mkelfimage to make the image, pointed the Kconfig-based build at the elf image, and it all worked. This was on the Kontron board.
Further, you should in the Kconfig-based build just be able to point coreboot at a kernel elf image (vmlinux, say) and it should work. Only big issue is that linuxbios was designed from 1999 on to enter at the 32-bit entry bit, not the 16-bit entry point. It's always been pretty easy to do, however, since from the very beginning linuxbios/coreboot had "load a kernel" as a primary goal.
ron