esp32: Add some notes about the different CMake files.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

Author: projectgus

ports/esp32/CMakeLists.txt

@@ -1,5 +1,8 @@
 # Top-level cmake file for building MicroPython on ESP32.
-
+#
+# Note for maintainers: Where possible, functionality should be put into
+# esp32_common.cmake not this file. This is because this CMakeLists.txt file
+# needs to be duplicated for out-of-tree builds, and can easily get out of date.
 cmake_minimum_required(VERSION 3.12)
 
 # Retrieve IDF version

ports/esp32/esp32_common.cmake

@@ -1,3 +1,9 @@
+# This is the common ESP-IDF "main component" CMakeLists.txt contents for MicroPython.
+#
+# This file is included directly from a main_${IDF_TARGET}/CMakeLists.txt file
+# (or included from an out-of-tree main component CMakeLists.txt for out-of-tree
+# builds.)
+
 # Set location of base MicroPython directory.
 if(NOT MICROPY_DIR)
     get_filename_component(MICROPY_DIR ${CMAKE_CURRENT_LIST_DIR}/../.. ABSOLUTE)