Hide voltage warnings from previous boots

[mtlynch]

The voltage log collector shows voltage warnings that occurred at any time, but we really only care about the ones that happened on the current boot. It should be possible to filter the messages by changing the journalctl invokation to:

sudo journalctl --boot --lines=all -xe | grep -i "voltage"

We also might not need the -xe flag. I haven't tested whether it's necessary to get the voltage logs. If we do, we should switch to the verbose versions of the flags for better clarify:

-x --catalog Add message explanations where available
-e --pager-end Immediately jump to the end in the pager

Testing note

An easy way to force TinyPilot to emit a low voltage warning is to connect its power cable to a PC's USB port instead of a real 3 Amp USB adaptor.