convert lvgl from submodule to a plain old directory

lib/lvgl/tests/makefile/Makefile

@@ -0,0 +1,9 @@
+LVGL_DIR := $(CURDIR)/../..
+include ../../lvgl.mk
+
+CSRCS += test.c
+CFLAGS += -DLV_CONF_SKIP=1 -DLV_BUILD_TEST=1 -I$(LVGL_DIR)/..
+COBJS := $(patsubst %.c, %.o, $(CSRCS))
+
+test_file: $(COBJS)
+	$(CC) -o $@ $^

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