feat(littlefs): add lv_fs_littlefs system as a driver (#4677)

Co-authored-by: Gabor Kiss-Vamosi <kisvegabor@gmail.com>

docs/libs/fsdrv.md

@@ -2,14 +2,38 @@
 # File System Interfaces
 
 LVGL has a [File system](https://docs.lvgl.io/master/overview/file-system.html) module to provide an abstraction layer for various file system drivers.
+You still need to provide the drivers and libraries, this extension provides only the bridge between FATFS, LittleFS, STDIO, POSIX, WIN32 and LVGL.
 
-LVG has built in support for:
-- [FATFS](http://elm-chan.org/fsw/ff/00index_e.html)
-- STDIO (Linux and Windows using C standard function .e.g fopen, fread)
-- POSIX (Linux and Windows using POSIX function .e.g open, read)
-- WIN32 (Windows using Win32 API function .e.g CreateFileA, ReadFile)
+## Built in wrappers
 
-You still need to provide the drivers and libraries, this extension provides only the bridge between FATFS, STDIO, POSIX, WIN32 and LVGL.
+### FATFS
+
+Bridge for [FatFS](http://elm-chan.org/fsw/ff/00index_e.html). FatFS itself is not part of LVGL, but can be added and initialized externally.
+
+
+### LittleFS
+
+Though `lv_fs_littlefs` uses [LittleFS]((https://github.com/littlefs-project/littlefs)) API, the LittleFS library needs other external libraries that handle the mounting of partitions and low-level accesses, according to the given architecture. The functions for the latter are given to the lfs_t structure as pointers by an external low-level library.
+
+There's a convenience function called `lv_fs_littlefs_set_driver(LV_FS_LITTLEFS_LETTER, my_lfs)`, specific to `lv_fs_littlefs`, to attach a `lfs_t` object's pointer to a registered driver-letter. See its comments for more info.
+
+
+[esp_littlefs](https://components.espressif.com/components/joltwallet/littlefs) is a wrapper for LittleFS to be used in Espressif ESP-devices. It handles the mounting and has the low-level `littlefs_api` functions to read/write/erase blocks that LittleFS library needs. On mounting by `esp_littlefs` the `lfs_t` structures are created. You need to get a handle to these to use ESP with `lv_fs_littlefs`, as all functions use that `lfs_t` in LittleFS to identify the mounted partition. 
+
+
+In case you don't find a special function in the `lv_fs_littlefs` wrapper, you can look for it in the `esp_littlefs` API and use it directly, as `lv_fs_littlefs` and the `esp_littlefs` APIs can be used side-by-side. 
+
+### STDIO
+
+Bride to C standard functions on Linux and Windows. For example `fopen`, `fread`, etc.
+
+### POSIX 
+
+Bride to POSIX functions on Linux and Windows. For example `open`, `read`, etc.
+
+### WIN32 
+
+Bride to Win32 API function. For example `CreateFileA`, `ReadFile`, etc.
 
 ## Usage