Import Arduino.org boards variant

variants/arduino_mzero/debug_scripts/variant.gdb

...arduino_mzero/linker_scripts/gcc/flash_with_bootloader.ld

...uino_mzero/linker_scripts/gcc/flash_without_bootloader.ld

variants/arduino_mzero/openocd_scripts/arduino_zero.cfg

variants/arduino_mzero/pins_arduino.h

variants/arduino_mzero/variant.cpp

variants/arduino_mzero/variant.h