More robust start time setting, 1Hz interrupt handling

Two big internal changes meant to improve robustness:

1. New sequencing on initial start time setting. We take an extra second to explicitly charge the RTC backup capacitor at full voltage before clearing the low voltage flag, then we execute a software interrupt to re-run normal reset checks try to catch any problems quickly and definitively.

2. New filter/interlock strategy for processing the FOUT signal from the RTC to avoid extra steps on spurious interrupts.

Both of these strategies slightly increase power usage, and the filtering increases it over the entire lifetime. We can mitigate some of this extra drain by using a virtual port to access the FOUT signal.

We also update the delay in burnthis.bat to reflect the additional time it takes charge the RTC backup cap before we set the new time.