[coreboot] Verifiable UEFI OS

[CJ]

First, I just want to thank all of you for what you do.  Using Coreboot is
delightful.  I had a question about how the program verifies UEFI OS's.
I'm using it to update a chromebook to ChromeOS flex and receiving the
error; doesn't contain a verifiable 64-bit UEFI OS.d From the documentation
ChromeOS Flex is supposed to be a 64 bit UEFI so I"m wondering if it could
be something I did while creating the boot media, a problem with updating
chromebook, or how coreboot is verifying the OS?  After entering the menu I
can boot to the USB and install just fine so no big deal just curious why
it gave that error.  Thanks again for putting in the work!  We all
appreciate it.