Merge branch 'feature/init_refactoring' into 'master'

Startup flow refactoring

This set of commits changes the startup code in a way that lets the application choose if/when to initialize WiFi/BT.

Application entry point is now a more familiar `main()` function. This function is executed in its own task. Application may choose to do some initialization from main function, create some tasks and then return from `main`. Simple applications may choose to do all their work from `main`.

Additionally this MR splits event handling code into two parts.

- One part is a set of standard handlers for WiFi and DHCP events. Most applications will use this set of handlers, and it is made available via new `esp_event_process_default` function.
- Another part is the default implementation of event handling loop. Some applications may choose to use default event loop through `esp_event_loop_` set of APIs, which start an event handling task and call user-provided event callback from this task. Other applications may create an event queue and implement event loop themselves. In this case application has to provide `esp_event_send` function. In this case the implementation provided by `esp_event_loop_` module is unused.

esp-idf-template has been updated to match this set of changes: https://github.com/espressif/esp-idf-template/tree/feature/init_refactoring
BT example has also been updated.

We need to provide examples of both event handling approaches. This will be done in a separate follow-up MR.

See merge request !112