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/esp8266/rom_functions.h

@@ -0,0 +1,17 @@
+#ifndef _ROM_FUNCTIONS_H
+#define _ROM_FUNCTIONS_H
+
+#include <stdint.h>
+
+uint32_t Wait_SPI_Idle();
+
+void uart_div_modify(uint32_t uart_no, uint32_t baud_div);
+
+void ets_delay_us(uint32_t us);
+int ets_printf(const char *fmt, ...) __attribute__ ((format (printf, 1, 2)));
+
+void system_soft_wdt_feed();
+
+void Cache_Read_Enable_New();
+
+#endif