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/esp8266/include/driver/spi_interface.h

@@ -126,8 +126,6 @@ typedef enum
     SpiPinCS_2 = 2,
 } SpiPinCS;
 
-#pragma pack (1)
-
 /**
  * @brief SPI attribute
  */
@@ -152,8 +150,6 @@ typedef struct
     uint8_t     dataLen;        ///< Data byte length.
 } SpiData;
 
-#pragma upack (1)
-
 #define SHOWREG() __ShowRegValue(__func__, __LINE__);
 
 /**
@@ -325,4 +321,4 @@ void SPIIntClear(SpiNum spiNum);
 }
 #endif
 
-#endif //  __SPI_INTERFACE_H__
+#endif //  __SPI_INTERFACE_H__