From https://chromium-review.googlesource.com/#/c/423460/3/src/vboot/vbnv_cmos.c , Aaron recommends in read_vbnv_cmos, 

"I think that we should provide the rtc_fail to a variant of this function such that we always use flash if rtc_fail is set. However, init_vbnv_cmos() is way late if we were wanting to ensure we had up to date vbnv in the verstage path.

Somehow we should be able to query for the RTC fail state so we don't just rely on the crc. That's orthogonal to this change, but something we should think about."