Added printf to Print class to have Serial.printf() functionality

[beegee-tokyo]

Adafruits version of the SAMD Arduino framework has Print::printf function. As the SAMD MCUs are not as memory limited than the AVR 8 bit MCUs I think it will not hurt to add it.

[matthijskooijman]

Thanks for your contribution. However, I'm closing this, since:

• Changes to Print.h should be made through the arch-independent https://github.com/arduino/ArduinoCore-API repo in the future
• There is already a similar PR there: Add printf to print class ArduinoCore-API#28
• So far, the Arduino team has specifically decided against including printf, see Print.h add printf API ArduinoCore-API#32 and Add printf to print class ArduinoCore-API#28 (comment)