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Please verify as a reference that you are following the procedure stated on page 19 of the Apollolake implementation.
By the way, please review as a reference the information stated at the Patch set updated for coreboot: ifwitool: Add new tool for managing IFWI images and the following website: https://review.coreboot.org/#/c/14896/
It is important to let you know that the coreboot consultations should be addressed as a reference at the channels listed in the coreboot consulting services website.
We hope that this information may help you.