Support for upload Files for LittleFS or SPIFFS on esp

[marelab]

It would be a great addition if it would possible to integrate the file upload for Spiffs and LittleFS to the spiffs area of the esp. It is mostly overkill to do it over the external SD Card. But maybe a lot of the work for the SD card could be reused just by adding a select flag to upload to SD or SPIFF and integrate the upload tool for the esp in the buildchain when the flag is set. That would make a lot of fancy web apps possible that could host html img and css files.