NOTE: this was developed against v1.23 tag, but rebased to master to push. It was not further tested on master.

This looks OK to me, although I now wonder why other files in the main_esp32xx directories are found OK when building out-of-tree.

For in-tree builds, the IDF project's "main" component is set to ports/esp32/main_${IDF_TARGET} and this directory contains target-specific idf_component.yml, CMakeLists.txt and linker.lf files which are picked up by the common idf_component_register function.

For the out of tree build example in micropython-example-boards, the IDF project's main component is boards/CUSTOM_ESP32/main. This directory currently only contains idf_component.yml, CMakeLists.txt because (as you point out) there was no linker fragment at the time it was last updated.

Probably the linker fragment should be copied into the out-of-tree main component rather than hard-coded to be in-tree. This will allow out-of-tree builds to set custom linker fragments, which is probably useful sometimes.

How does that sound, @karlp?

(When discussing this with @dpgeorge we also noted that there are some other recent changes in the CMakeLists.txt file that aren't yet reflected in the out-of-tree copy. Will fix.)

Probably the linker fragment should be copied into the out-of-tree main component rather than hard-coded to be in-tree

I tested that and it works.

Perhaps, we could default to using the existing in tree linker fragments, but make it easy and clear how to override it with a local one? I'm not a uge fan of copying more into the "external boards" repo, as it's just yet more stuff that needs to be maintained and kept in sync. Especially when, at present, only esp32 classic needs a special linker fragment. the other esp32 parts are all just specifying an empty file to match the requirements. I'd like to keep it feasiible to "easily" add micropython as a submodule, rather than having forks of the main repo be the most tempting path.
I'll work up a new version for review.

Closing in favour of #16658.