Switch to LittleFS

[pzbitskiy]

Replacing SPIFFS by LittleFS. No more compilation warnings.

Tested on Wemos D1 Mini:

1. PlatformIO IDE both image and FS upload
2. OTA both image and FS upload

[dzungpv]

Because this project also support ESP32 and Arduino IDE, so I need to test this PR before merge, and what about OTA from SPIFFS to LittleFS or must be fresh install?

[pzbitskiy]

I did firmware OTA update first and then partition OTA update.
I thought about some kind of data migration but have not found a way to read the entire partition from firmware - idk, not very familiar with micro controllers programming.
In theory it is possible to have both SPIFFS and LittleFS support gated by #define SPIFFS_LITTLEFS_MIGRATION macro so that it tries SPIFFS first and reads files to memory (assuming ~48K is enough for all config files), then formats it to LittleFS and writes files down. Does not look too complex, maybe I'll have time to implement this next months.