convert lvgl from submodule to a plain old directory

lib/lvgl/CMakeLists.txt

@@ -0,0 +1,17 @@
+cmake_minimum_required(VERSION 3.12.4)
+
+if(NOT ESP_PLATFORM)
+  project(lvgl HOMEPAGE_URL https://github.com/lvgl/lvgl)
+endif()
+
+set(LVGL_ROOT_DIR ${CMAKE_CURRENT_LIST_DIR})
+
+if(ESP_PLATFORM)
+  include(${CMAKE_CURRENT_LIST_DIR}/env_support/cmake/esp.cmake)
+elseif(ZEPHYR_BASE)
+  include(${CMAKE_CURRENT_LIST_DIR}/env_support/cmake/zephyr.cmake)
+elseif(MICROPY_DIR)
+  include(${CMAKE_CURRENT_LIST_DIR}/env_support/cmake/micropython.cmake)
+else()
+  include(${CMAKE_CURRENT_LIST_DIR}/env_support/cmake/custom.cmake)
+endif()