You can enable CONFIG_HAVE_EC_BIN and define CONFIG_EC_BIN_PATH (https://review.coreboot.org/plugins/gitiles/coreboot/+/refs/heads/master/src/southbridge/intel/common/firmware/Kconfig#115) to point to the EC binary and the build system will use ifdtool to inject the binary at the EC region that is defined in the descriptor. (https://review.coreboot.org/plugins/gitiles/coreboot/+/refs/heads/master/src/southbridge/intel/common/firmware/Makefile.inc#64)

If you are creating a descriptor layout from scratch you will need to define the EC region in FLREG8: (example from google/drallion mainboard, which matches the layout in chromeos.fmd)
00000000:00000fff fd
00438000:01ffffff bios
00101000:00433fff me
00434000:00437fff pd
00001000:00100fff ec

I have the binary file for the original system BIOS for this platform and so have manage to use ifdtool to extract the 4KB flash descriptor which I am in the process of adding to the image with CONFIG_HAVE_IFD_BIN and CONFIG_IFD_BIN_PATH. Does that save me having to worry about anything other than defining the CONFIG_HAVE_EC_BIN and CONFIG_EC_BIN_PATH values?