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/driver/spi_interface.c

@@ -24,6 +24,7 @@
 #include "esp8266/ets_sys.h"
 #include "esp8266/pin_mux_register.h"
 #include "esp_libc.h"
+#include "freertos/portmacro.h"
 //*****************************************************************************
 //
 // Make sure all of the definitions in this header have a C binding.
@@ -457,7 +458,7 @@ int ICACHE_FLASH_ATTR SPIMasterRecvStatus(SpiNum spiNum)
 
     while (READ_PERI_REG(SPI_CMD(spiNum))&SPI_USR);
 
-    uint8_t data = (uint8)(READ_PERI_REG(SPI_W0(spiNum)) & 0xff);
+    (void)(READ_PERI_REG(SPI_W0(spiNum)) & 0xff);
     SHOWREG();
 
     return (uint8)(READ_PERI_REG(SPI_W0(spiNum)) & 0xff);
@@ -520,4 +521,4 @@ void ICACHE_FLASH_ATTR SPIIntClear(SpiNum spiNum)
 
 #ifdef __cplusplus
 }
-#endif
+#endif