PCR documentation usage changes for coreboot 4.8.1 and newer versions (…

…#42)

* Update Keys.md

Documentation changes linked to PCR usage in coreboot 4.8.1 and newer versions, to be applied at the same time as linuxboot/heads#793 is merged.
Linked to VBOOT+Measured boot/Measured boot changes applied directly in coreboot so that we have a common base prior of going linuxboot/heads#709 and linuxboot/heads#721 

@PatrickRudolph comments welcome

* Update Keys.md

Removed comment about MRC needing to be measured into different PCR since coreboot is merging them all under PCR2 in newer VBOOT+measured boot/measuredboot
Ref:https://doc.coreboot.org/security/vboot/measured_boot.html

Keys.md

@@ -94,13 +94,13 @@ The actual assignment needs to be updated in the code; there are outstanding iss
 [SMM reloc](https://github.com/osresearch/heads/issues/13)
 ) that need to be resolved as well.  Until then this is a rough draft of how Heads uses the TPM PCRs.
 
-0: Boot block
+0: Nothing for the moment
 
-1: ROM stage
+1: Nothing for the moment
 
-2: RAM stage (and MRC? this should be separate)
+2: Boot block, ROM stage, RAM stage, Heads linux kernel and initrd
 
-3: Heads Linux kernel and initrd
+3: Nothing for the moment
 
 4: Boot mode (0 during `/init`, then `recovery` or `normal-boot`)