ESP32: Support for external components & modules

[jmattsson]

Addresses comments in #3397

• [x] This PR is for the dev branch rather than for the release branch.
• [x] This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• [x] I have thoroughly tested my contribution.
• [x] The code changes are reflected in the documentation at docs/*.

With the switch to IDFv4, the old extmod support became defunct, and we've lacked an ability to easily have external modules. This PR builds on modifications used at my $work, where we do have that need. The functional change is simply taking EXTRA_COMPONENT_DIRS from the environment and injecting it into the IDF, letting it do it's thing (since extra component support is already baked into it). Additionally I've included a template here as a quick-start, and added some basic documentation to hopefully give people the information needed to get off the ground.

I also fixed up some old esp866'isms that I found in the nodemcu-pil.md documentation.

@tomsci this is for you. @jpeletier you may want to have a look at this; it is not as neat as your extmod stuff, but on the other hand it leaves the heavy lifting to the IDF and is likely to stay supported longer.

Feedback and improvements welcome.

[tomsci]

Overall I think the approach is a good one 👍 Do you have any suggestion for how to deal with having multiple EXTRA_COMPONENT_DIRS set? I have two separate external modules to include (each is a separate git repo) each with a separate Kconfig file, and as I've initially set them up based on the example, I have 2 separate "External modules" menu entries under "Component config". Which works, but is inelegant. I guess maybe don't put a menu "External modules" in my Kconfigs and let those 2 modules' config options both be directly under "Component config"?

[jmattsson]

Hmm, that is a good point. I forgot that Kconfig doesn't merge menus with the same name. Right now I can't think of an elegant way around it (that isn't fighting against the IDF structure lots). What you suggest is probably the least ugly option.

[jmattsson]

Alright, @tomsci I've pushed an updated version here, heavily based on your suggestions. After thinking it through, I agree that trying to use the same mechanism for the NodeMCU modules directory and external modules is not worth it (and possibly even a bad idea).

I've added two functions - extmod_register_conditional(MODNAME) and extmod_register_unconditional(MODNAME). The former for when there is a Kconfig option for toggling the module on/off, and the latter for when there isn't. The mere action of listing the module dir in EXTRA_COMPONENT_DIRS could be considered enough for some some people to want a module automatically enabled, so it seemed worth providing the option.

[jmattsson]

Merging in the absence of further feedback. Happy to refine later if needed.