AdjustRTC at Compile Time Requires Clearing Cache

[mattgaidica]

Cached compilations can result in stale DATE and TIME ∴ requires pressing shift + upload in Arduino IDE. @KravitzLab

The routine won't update the RTC if it has been previously set and recognizes a cached compile time (it stores a compilation datetime string in non-volatile memory using "preferences"). Still, if there is a more recent (including a cached) compilation, the RTC could reflect the cached compilation datetime. Options:

1. The flasher needs to remember to use shift + upload.
2. RTC setting uses a different script, but flasher STILL needs to shift + upload. This is kind of dumb and painful (To flash twice) but it might reduce the possibility of error. You would need to disable adjustRTC() in begin.
3. Fancier stuff: Hublink RTC, standalone python script to connect to port and serialize datetime, etc.

[KravitzLab]

We do the annoying #2 option with FED3 :)

The other option (which we also do with FED3) is to put a time-setting mode on the device itself - that can be a convenient back stop for any clock adjustments. I'll open an issue for that and knock it out at some point.