Field updates to firmware and web manager partition via SD card

[selimnairb]

Original report by Brian Miles (Bitbucket: [Brian Miles](https://bitbucket.org/Brian Miles), ).

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Users need an easy way to field update WIBL logger firmware and web manager partition via SD card. This could work by manually storing the update on an SD card (possibly the same card where logger files are stored) that can be checked on boot and enter an update process if present. This could also work via the webserver, where the user could upload the new firmware via the web interface, which would save it to the SD card. Then on next boot the updates would be applied in the same manner as if the files were manually placed on the SD card.

[selimnairb]

Original comment by Brian Miles (Bitbucket: [Brian Miles](https://bitbucket.org/Brian Miles), ).

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This appears to be a relevant minimal working example to update the firmware: https://github.com/espressif/arduino-esp32/blob/master/libraries/Update/examples/SD_Update/SD_Update.ino

[selimnairb]

Original comment by Brian Miles (Bitbucket: [Brian Miles](https://bitbucket.org/Brian Miles), ).

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For updating the website on the filesystem, we should be able to read from a zipfile on the SD card (say wibl-web-update.zip) using something like unzipLIB and then use the built-in FS library to delete what is there and then write the contents of the zipfile.