Renz Bagaporo 98dc1b0188 esp_system: introduce intermediary function to call after system init
This MR uses an intermediary function `start_app` to call after system
initialization instead of `app_main`.

In RTOS builds, freertos provides `start_app` and calls `app_main`.
In non-RTOS builds, user provides `start_app` directly.
2020-06-19 18:40:10 +10:00
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