This change reduces chances that a large .bss segment will push .data all the way into
0x3ffe1320 — 0x3ffe5320 range where the bootloader stack is, creating a problem when
bootloader will be loading application into memory.
With this change, .data would need to be at least 200k big to cause problems.
Deep sleep: Any source named rtc_wake_stub* is linked as RTC wake stub code
Also move esp_deepsleep.h documentation out to docs/deep-sleep-stub.rst
See merge request !142
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.
