[vpd] implement safety checks for RO and RW vars

[insomniacslk]

A desired feature may be to enforce that RO vars always take precedence over RW vars. Currently we do the opposite: a RW vars overrides a RO one with the same name.

We may:

• Invert the check to give priority to RO vars
• Implement facilities to erase RW vars for which a RO vars with the same name exists

[insomniacslk]

As discussed, we will:

• make all the BootNNNN variables read-write
• introduce BootFailSafeNNNN read-only variables, to call only if the BootNNNN vars fail to boot

Additionally, implement redundancy (since VPD doesn't do that), e.g. by introducing BootBackupNNNN