Merge branch 'zero' of github.com:arduino/ArduinoZero into zero

cores/arduino/Arduino.h

cores/arduino/USB/HID.cpp

cores/arduino/USB/USBAPI.h

cores/arduino/USB/USBCore.cpp

cores/arduino/USB/USB_device.h

cores/arduino/USB/USB_host.h

cores/arduino/USB/USB_interrupt.c

cores/arduino/USB/samd21_device.c

cores/arduino/USB/samd21_device.h

cores/arduino/USB/samd21_host.c

cores/arduino/USB/samd21_host.h

cores/arduino/wiring.c

...s/validation/validation_usb_device/build_as6/test.cppproj

cores/validation/validation_usb_host/build_as6/test.atsln

cores/validation/validation_usb_host/build_as6/test.cppproj

cores/validation/validation_usb_host/pgmstrings.h

cores/validation/validation_usb_host/test_USB_desc.cpp

.../validation_usb_host/test_usb_host_KeyboardController.cpp

...ion/validation_usb_host/test_usb_host_MouseController.cpp

cores/validation/validation_usb_host/test_usb_host_adk.cpp