Has anyone done a Configuration File such as .ini or .json on an SD Card?
On a recent Particle marketing video they mention Ota Config Files but a search of the website only turned up a 'coming soon'.
The goal is to have one firmware file but the many config options such as IP, Cell, User choices, etc are all read from the config file on the SD card.
Hopefully someone has already invented this wheel. 
The recommended way of storing configuration is using Ledger. That's the page for using Ledger for configuration.
It will automatically synchronize betweem the cloud and device, and you don't need an SD card as the data is stored in the internal flash memory. The data is stored as JSON.
Thanks. Yet another new topic to study. 
I've been reading up on Ledger. So is this still a good solution if we have 200 or more devices each with a unique configuration?
Yes, it will scale up to any number of devices. You can include a organization-wide, product-wide, or per device configurations, or you can have all three and merge the results to do overrides.
@pchidley I can confirm that Ledger method works well, if you search for post on this same topic you will see another idea from @rickkas7 which is to create a device to cloud ledger so that you can track from the console when a device has picked up and used the cloud configuration data. I use this to manage versions. I have only implemented device specific configuration because after analysis it became clear all but one setting was device specific. One little wrinkle has been editing the JSON file to copy into the Ledger cloud instance - the double quotes "" around some items become "" not sure why but the 'save' button isn't enabled until everything is correct.