Add SPIFFS.unmount() for use in OTA.onEnd()

[andig]

Use case: allow to store configuration from ram back to freshly updated SPIFFS. Avoid loss on configuration due to OTA wiping the filesystem config files.

Potential flow:

// OTA update overwrites SPIFFS and unmounts the SPIFFS library

ArduinoOTA.onEnd([]() {
  if (SPIFFS.begin()) {
    // write config files to spiffs again
  }
});

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[andig]

Great stuff, thank you! Please feel free to close.

[andig]

@igrr once OTA finishes I'm assuming that SPIFFS already is unmounted and I'd need to wrap further writes into SPIFFS begin/end. Is this correct?