convert lvgl from submodule to a plain old directory

lib/lvgl/tests/makefile/test.c

@@ -0,0 +1,9 @@
+#if LV_BUILD_TEST
+#include <stdio.h>
+#include "lvgl/lvgl.h"
+
+int main(void) {
+    lv_init();
+    return 0;
+}
+#endif