convert lvgl from submodule to a plain old directory

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diff --git a/lib/lvgl b/lib/lvgl
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-Subproject 0732400e7b564dd0e7dc4a924619d8e19c5b23a
diff --git a/lib/lvgl/docs/porting/tick.md b/lib/lvgl/docs/porting/tick.md
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+++ b/lib/lvgl/docs/porting/tick.md
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+# Tick interface
+
+LVGL needs a system tick to know elapsed time for animations and other tasks.
+
+You need to call the `lv_tick_inc(tick_period)` function periodically and provide the call period in milliseconds. For example, `lv_tick_inc(1)` when calling every millisecond.
+
+`lv_tick_inc` should be called in a higher priority routine than `lv_task_handler()` (e.g. in an interrupt) to precisely know the elapsed milliseconds even if the execution of `lv_task_handler` takes more time.
+
+With FreeRTOS `lv_tick_inc` can be called in `vApplicationTickHook`.
+
+On Linux based operating systems (e.g. on Raspberry Pi) `lv_tick_inc` can be called in a thread like below:
+```c
+void * tick_thread (void *args)
+{
+      while(1) {
+        usleep(5*1000);   /*Sleep for 5 millisecond*/
+        lv_tick_inc(5);      /*Tell LVGL that 5 milliseconds were elapsed*/
+    }
+}
+```
+
+
+
+## API
+
+```eval_rst
+
+.. doxygenfile:: lv_hal_tick.h
+  :project: lvgl
+
+```