Add api to get CPU temperature

Fixes: https://github.com/espressif/arduino-esp32/issues/563
Fixes: https://github.com/espressif/arduino-esp32/issues/625
Source:
https://github.com/pcbreflux/espressif/blob/master/esp32/arduino/sketchb
ook/ESP32_int_temp_sensor/ESP32_int_temp_sensor.ino

cores/esp32/esp32-hal.h

@@ -61,6 +61,9 @@ void yield(void);
 #include "esp32-hal-bt.h"
 #include "esp_system.h"
 
+//returns chip temperature in Celsius
+float temperatureRead();
+
 unsigned long micros();
 unsigned long millis();
 void delay(uint32_t);