fix: Fixing lots of compilation warnings

- fix(esp8266):
  - Adding includes for missing symbols.
  - Removing unused variables.
  - Skip unsupported packing pragmas.
  - Add rom_functions.h for symbols that come from the ESP ROM. Add attributes on
ets_printf so GCC will check the syntax of the formatting string and types of
the arguments.
  - Add ETS_GPIO_INTR_EN(DIS)ABLE macro.
  - Use gpio_output_conf instead of gpio_output_set.

- fix(freertos):
  - Define functions that are useful.
  - Use correct printf symbols when printing.

- fix(lwip):
  - Ignore the warning in sntp.

- fix(mqtt):
  - `xTicksToWait` is unsigned, can't check for less than zero. Remove
unused variables.

- fix(newlib):
  - `_free_r()` returns `void`, not `void *`.
  - Adding includes for missing symbols.

- fix(ssl):
  - Make sure functions always return a value.

Merges https://github.com/espressif/ESP8266_RTOS_SDK/pull/188

components/mqtt/paho/MQTTClient-C/src/FreeRTOS/MQTTFreeRTOS.c

@@ -68,7 +68,7 @@ void TimerCountdown(Timer* timer, unsigned int timeout)
 int TimerLeftMS(Timer* timer)
 {
     xTaskCheckForTimeOut(&timer->xTimeOut, &timer->xTicksToWait); /* updates xTicksToWait to the number left */
-    return (timer->xTicksToWait < 0) ? 0 : (timer->xTicksToWait * portTICK_RATE_MS);
+    return timer->xTicksToWait * portTICK_RATE_MS;
 }
 
 
@@ -90,7 +90,6 @@ static int esp_read(Network* n, unsigned char* buffer, unsigned int len, unsigne
     portTickType xTicksToWait = timeout_ms / portTICK_RATE_MS; /* convert milliseconds to ticks */
     xTimeOutType xTimeOut;
     int recvLen = 0;
-    int recv_timeout = timeout_ms;
     int rc = 0;
 
     struct timeval timeout;
@@ -128,7 +127,6 @@ static int esp_write(Network* n, unsigned char* buffer, unsigned int len, unsign
     portTickType xTicksToWait = timeout_ms / portTICK_RATE_MS; /* convert milliseconds to ticks */
     xTimeOutType xTimeOut;
     int sentLen = 0;
-    int send_timeout = timeout_ms;
     int rc = 0;
     int readysock;