Merge branch 'zero-core' into zero-package

boards.txt

bootloaders/zero/drivers/cdc_enumerate.c

bootloaders/zero/main.c

bootloaders/zero/samd21_sam_ba.bin

cores/arduino/SERCOM.h

cores/arduino/Stream.cpp

cores/arduino/USB/USBCore.cpp

cores/arduino/Uart.cpp

cores/arduino/Uart.h

cores/arduino/WVariant.h

cores/arduino/startup.c

cores/arduino/wiring_analog.c

libraries/SPI/SPI.h

platform.txt

programmers.txt

.../arduino_zero/linker_scripts/gcc/flash_with_bootloader.ld

variants/arduino_zero/variant.cpp

variants/arduino_zero/variant.h