Configurable maxage for SondeHub

[ae5au]

Added configurable parameter replacing "define SONDEHUB_MAXAGE 15". This is particularly helpful for the DFM radiosondes and emulates the guidance that projecthorus gives for auto_rx.

https://github.com/projecthorus/sondehub-infra/wiki/DFM-radiosonde-above-1000-and-not-enough-data-to-perform-z-check

Ideally, the logic would be to submit every 10 valid decoded packets whether that took 10 seconds or a minute (maybe with a max timeout) when decoding a DFM sonde, but that's beyond my comfort with the codebase at this point.

I did set a minimum of 15 seconds in case someone updates the firmware without updating the filesystem. If that should be handled differently, feel free to update or let me know how you'd like it fixed.

[ae5au]

When looking at other PRs, I just noticed that there is a version for the filesystem in version.h. I'll leave it as-is and let whomever merges set it to the next version at that time.