Add short log cache to local disk for BEEs and SMBs

Both the BEE and the SMB (temperature board) log to the network. The BEE can survive a network outage for a while, and the SMB is expected to run "as long as possible" to keep the detectors warm during an emergency warmup, even (or especially) when it is disconnected from the outside world.

Both also have local storage. They should log to that well enough for postmortems should they become disconnected.