Don't attempt to save NodeDB on low-batt shutdown to prevent FS corruption

[thebentern]

Addresses part of #5259 by skipping the NodeDB save on low battery shutdown. This should result in minimal loss of data, because the NodeDB is saved in other domain events within the firmware regularly. Plus in any regard, it's better than potentially losing the whole enchilada.

@todd-herbert we can collaborate on this branch to try to figure out the low voltage warning startup halt, or we work that separately and push this one forward.
I was researching the best ways to persist the status that we shut down due to low battery, so that we can check attempt to run some ADC samples on the next boot. It looks like RTC slow memory could be an option: https://docs.espressif.com/projects/esp-idf/en/stable/esp32/api-guides/memory-types.html#rtc-slow-memory

[thebentern]

Also, I'm going to get a harness setup for my PSU so that I can reproduce this condition on demand hopefully

[dm5tt]

Reproducing it shouldn't be too complicated.

You can start with pulling and plugging the power cable manually into the PSU out and in fast trying to abort the early boot-up sequence.

[todd-herbert]

@dm5tt Do you feel that the corruption is happening on boot, rather than during low power shutdown?

[todd-herbert]

@thebentern I actually hadn't been working on this separately just yet. I got a bit sidetracked looking into whether the battery % reporting is linear enough. It's been a bit of a slow process testing that one.

I think this PR more than makes sense on its own. Catching low voltage at boot can be another job for the future.