[coreboot] Understand vboot logic to select FW_B

Folks,
I'm working on coreboot on ARMv8 architecture and recently attempted to 
verify FW_B can be selected if FW_A is corrupted.

In reviewing the code-path through coreboot vboot wrapper and 
vboot_reference library I'm not understanding the logic.

The logic appears to be that vboot_reference library determines FW_A is 
corrupt and forces a reboot.

After reboot, early in verstage_main(), both the NVRAM (flash) and 
vb2_context structures are initialized, wiping out any history of 
failure condition.

I know I must be missing something in my analysis but don't see it.

Can anybody on this list share their experiences in confirming the FW_B 
boot path if FW_A is corrupt?
Cheers,
T.mike