esp32: Bootloader wake deep sleep stub

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.

components/esp32/include/esp_system.h

@@ -62,21 +62,6 @@ void system_restore(void);
   */
 void system_restart(void);
 
-/**
-  * @brief     Set the chip to deep-sleep mode.
-  *
-  *            The device will automatically wake up after the deep-sleep time set
-  *            by the users. Upon waking up, the device boots up from user_init.
-  *
-  * @attention  The parameter time_in_us to be "uint64" is for further development. 
-  *                  Only the low 32 bits of parameter time_in_us are avalable now.
-  *
-  * @param     uint64 time_in_us : deep-sleep time, only the low 32bits are avalable now. unit: microsecond
-  *
-  * @return    null
-  */
-void system_deep_sleep(uint64_t time_in_us);
-
 /**
   * @brief  Get system time, unit: microsecond.
   *