1. Add detailed return error code for wifi APIs
2. Add description about error code in esp_wifi.h
3. Modify esp_wifi_reg_rxcb to esp_wifi_internal_reg_rxcb
4. Modify esp_wifi_set_sta_ip to esp_wifi_internal_set_sta_ip
5. Mark system_init as deprecated API
This change separates definitions in esp_event.h and functions in event.c into several parts:
- event structure definitions (esp_event.h)
- default implementations of event handlers (event_default_handlers.c)
- default implementation of event loop (event_loop.c, esp_event_loop.h)
Purpose of this change is to allow applications choose their own poison:
- full control of event loop at the expense of more bootstrap code
- pre-defined event task firing event callbacks, but less code in app_main.c
