This adds the following APIs to enable various wakeup sources:
- esp_deep_sleep_enable_timer_wakeup
- esp_deep_sleep_enable_ulp_wakeup
- esp_deep_sleep_enable_ext0_wakeup
- esp_deep_sleep_enable_ext1_wakeup
And an API to start deep sleep:
- esp_deep_sleep_start
App can contain a stub program resident in RTC fast memory. Bootloader
will load the stub on initial boot. If the device wakes from deep sleep,
the stub is run immediately (before any other data is loaded, etc.)
To implement a custom wake stub, implement a function in your program:
```
void RTC_IRAM_ATTR esp_wake_deep_sleep(void)
{
esp_default_wake_deep_sleep();
// other wake logic
}
```
... and it will replace the default implementation.
