[FEATURE] Add integration with external programs/projects the right way

[acassis]

Is your feature request related to a problem? Please describe.

Currently there is possible to add external applications on nuttx-apps creating an "external/" directory inside apps/ [1]. However it expects that the added project has the right files inside it (Kconfig, Make.defs, Makefile, CMakelist.txt, etc).

Others RTOSes like zephyr has a similar solution (although they used a confuse name "Module" and that is a bad name since for a kernel a module means dynamic loadable component). Also in their case they expects a directory called "zephyr" inside the target project. Some examples:
https://github.com/open-quantum-safe/liboqs
https://github.com/civetweb/civetweb/tree/master/zephyr
https://github.com/lvgl/lvgl/tree/master/zephyr

This is more than a better solution, this is their way to spread their naming/awareness around all other projects in the Internet and avoids the process of creating a download script in their repository, as we do for LVGL and others.

There is a complete (and verbose) documentation about it here: https://docs.zephyrproject.org/latest/develop/modules.html

1 - It is a hidden feature, there is no Documentation about it: https://nuttx.apache.org/docs/latest/search.html?q=external&check_keywords=yes&area=default

Describe the solution you'd like

We could follow the same approach, creating a "nuttx" or "NuttX" repository inside other projects simplify the integration process.

Describe alternatives you've considered

No response

Verification

• I have verified before submitting the report.

[iamadahn]

I was just passing by, but that a great feature to improve developer experience imho. Personally I really like how it is done in zephyr, but nuttx way with makefiles and building stuff inside nuttx main repo, while developing your app in another directory which also should be located (or symlinked) inside another repo (nuttx-apps) and this ton of object files that will be everywhere, unfortunately all of that just really puts off. I know that cmake kinda solves that problems and I was able to make completely out-of-tree builds with that and nuttx cmake support is great besides documentation, but still there are a lot of boards that doesn't support it, like all esp32 stuff, so you are still tied to that old-fashioned all-in-tree way. Excuse me for that stream of consciousness, I'm just really hyped for that feature

[acassis]

@iamadahn don't worry, other comments reflect the currently reality. About the CMake you can see by the history that @simbit18 is improving it day by day, so soon all boards and apps will be supported.

About ESP32 family I think it is not complicated to add support since everything is similar to other boards (except for the HAL, that although it not the default porting way on NuttX, makes it easier to Espressif to add support to new features as chips).