lib_max6675: Read temperature from MAX6675 thermocouple converter.

The MAX6675 is a device that converts the output of a K type thermocouple into a binary temperature. It returns a value from 0C to 1023.75C as a 12 bit binary value. A return value of MAX6675_ERROR indicates a wiring problem.

void max6675_init(void);
int16_t max6675_read(void); /* Get temperature. out: 12-bit signed value, degrees C * 4 */