`rendmp` should be usable on Sidecars

[Aaron-Hartwig]

There is a check which requires SPD data to be present, which we will only have on sleds: https://github.com/oxidecomputer/humility/blob/master/cmd/rendmp/src/lib.rs#L1629-L1670

aaron@sam ~ $ pfexec humility rendmp --rail V0P8_TF2_VDD_CORE --phase-check
humility: attached to 483:3754:0039001D4741500920383733 via ST-Link V3
humility: restoring default configuration...
humility: done restoring default configuration...
humility rendmp failed: member missing from struct: gimlet_bufs

We should make that check optional on Sidecar.

[Aaron-Hartwig]

I commented out the problematic lines of code above and I then hit another issue on sidecar:

$ pfexec ./humility rendmp --rail V0P8_TF2_VDD_CORE --phase-check
humility: attached to 483:3754:0039001D4741500920383733 via ST-Link V3
humility: restoring default configuration...
humility: done restoring default configuration...
humility rendmp failed: length mismatch between sensors and rails