This can be used to configure the ADC window interrupt on the SAMD21. It
uses OSCULP32K via GCLK6 to clock the ADC while in sleep mode (the same
as used for the EIC).
Note that attachAdcInterrupt()/detachAdcInterrupt() should be called
immediately before/after LowPower.sleep() otherwise analogRead() will
not work as expected.
There is also an example (AdcWakeup.ino) which is much like the
ExternalWakeup example but uses the ADC interrupt instead.
* Replace SerialUSB by SERIAL_PORT_USBVIRTUAL
Support boards where USB serial is named differently. E.g. the
Adafruit Feather M0 uses Serial1 as name for serial USB port.
Waking from deepSleep() is comparable to an hard reset; the sketch won't restart from the sleep invocation but the wakeuup source can be retrieved with wakeupReason()
