init lvgl code

LVGL.Simulator/lvgl/docs/overview/file-system.md

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+```eval_rst
+.. include:: /header.rst
+:github_url: |github_link_base|/overview/file-system.md
+```
+# File system
+
+LVGL has a 'File system' abstraction module that enables you to attach any type of file system.
+A file system is identified by an assigned drive letter.
+For example, if an SD card is associated with the letter `'S'`, a file can be reached using `"S:path/to/file.txt"`.
+
+## Ready to use drivers
+The [lv_fs_if](https://github.com/lvgl/lv_fs_if) repository contains prepared drivers using POSIX, standard C and the [FATFS](http://elm-chan.org/fsw/ff/00index_e.html) API.
+See its [README](https://github.com/lvgl/lv_fs_if#readme) for the details.
+
+## Adding a driver
+
+### Registering a driver
+To add a driver, a `lv_fs_drv_t` needs to be initialized like below. The `lv_fs_drv_t` needs to be static, global or dynamically allocated and not a local variable.
+```c
+static lv_fs_drv_t drv;                   /*Needs to be static or global*/
+lv_fs_drv_init(&drv);                     /*Basic initialization*/
+
+drv.letter = 'S';                         /*An uppercase letter to identify the drive */
+drv.cache_size = my_cache_size;           /*Cache size for reading in bytes. 0 to not cache.*/
+
+drv.ready_cb = my_ready_cb;               /*Callback to tell if the drive is ready to use */
+drv.open_cb = my_open_cb;                 /*Callback to open a file */
+drv.close_cb = my_close_cb;               /*Callback to close a file */
+drv.read_cb = my_read_cb;                 /*Callback to read a file */
+drv.write_cb = my_write_cb;               /*Callback to write a file */
+drv.seek_cb = my_seek_cb;                 /*Callback to seek in a file (Move cursor) */
+drv.tell_cb = my_tell_cb;                 /*Callback to tell the cursor position  */
+
+drv.dir_open_cb = my_dir_open_cb;         /*Callback to open directory to read its content */
+drv.dir_read_cb = my_dir_read_cb;         /*Callback to read a directory's content */
+drv.dir_close_cb = my_dir_close_cb;       /*Callback to close a directory */
+
+drv.user_data = my_user_data;             /*Any custom data if required*/
+
+lv_fs_drv_register(&drv);                 /*Finally register the drive*/
+
+```
+
+Any of the callbacks can be `NULL` to indicate that operation is not supported.
+
+
+### Implementing the callbacks
+
+#### Open callback
+The prototype of `open_cb` looks like this:
+```c
+void * (*open_cb)(lv_fs_drv_t * drv, const char * path, lv_fs_mode_t mode);
+```
+
+`path` is the path after the drive letter (e.g. "S:path/to/file.txt" -> "path/to/file.txt"). `mode` can be `LV_FS_MODE_WR` or `LV_FS_MODE_RD` to open for writes or reads.
+
+The return value is a pointer to a *file object* that describes the opened file or `NULL` if there were any issues (e.g. the file wasn't found).
+The returned file object will be passed to other file system related callbacks. (see below)
+
+### Other callbacks
+The other callbacks are quite similar. For example `write_cb` looks like this:
+```c
+lv_fs_res_t (*write_cb)(lv_fs_drv_t * drv, void * file_p, const void * buf, uint32_t btw, uint32_t * bw);
+```
+
+For `file_p`, LVGL passes the return value of `open_cb`, `buf` is the data to write, `btw` is the Bytes To Write, `bw` is the actually written bytes.
+
+For a template of these callbacks see [lv_fs_template.c](https://github.com/lvgl/lvgl/blob/master/examples/porting/lv_port_fs_template.c).
+
+
+## Usage example
+
+The example below shows how to read from a file:
+```c
+lv_fs_file_t f;
+lv_fs_res_t res;
+res = lv_fs_open(&f, "S:folder/file.txt", LV_FS_MODE_RD);
+if(res != LV_FS_RES_OK) my_error_handling();
+
+uint32_t read_num;
+uint8_t buf[8];
+res = lv_fs_read(&f, buf, 8, &read_num);
+if(res != LV_FS_RES_OK || read_num != 8) my_error_handling();
+
+lv_fs_close(&f);
+```
+*The mode in `lv_fs_open` can be `LV_FS_MODE_WR` to open for writes only or `LV_FS_MODE_RD | LV_FS_MODE_WR` for both*
+
+This example shows how to read a directory's content. It's up to the driver how to mark directories in the result but it can be a good practice to insert a `'/'` in front of each directory name.
+```c
+lv_fs_dir_t dir;
+lv_fs_res_t res;
+res = lv_fs_dir_open(&dir, "S:/folder");
+if(res != LV_FS_RES_OK) my_error_handling();
+
+char fn[256];
+while(1) {
+    res = lv_fs_dir_read(&dir, fn);
+    if(res != LV_FS_RES_OK) {
+        my_error_handling();
+        break;
+    }
+
+    /*fn is empty, if not more files to read*/
+    if(strlen(fn) == 0) {
+        break;
+    }
+
+    printf("%s\n", fn);
+}
+
+lv_fs_dir_close(&dir);
+```
+
+## Use drives for images
+
+[Image](/widgets/core/img) objects can be opened from files too (besides variables stored in the compiled program).
+
+To use files in image widgets the following callbacks are required:
+- open
+- close
+- read
+- seek
+- tell
+
+
+
+## API
+
+```eval_rst
+
+.. doxygenfile:: lv_fs.h
+  :project: lvgl
+
+```