Added support for MKRZero

boards.txt

bootloaders/mkrzero/samd21_sam_ba_arduino_mkrzero.bin

bootloaders/mkrzero/samd21_sam_ba_arduino_mkrzero.hex

bootloaders/zero/board_definitions.h

bootloaders/zero/board_definitions_arduino_mkrzero.h

bootloaders/zero/build_all_bootloaders.sh

variants/mkrzero/debug_scripts/variant.gdb

variants/mkrzero/linker_scripts/gcc/flash_with_bootloader.ld

...ts/mkrzero/linker_scripts/gcc/flash_without_bootloader.ld

variants/mkrzero/openocd_scripts/arduino_zero.cfg

variants/mkrzero/pins_arduino.h

variants/mkrzero/variant.cpp

variants/mkrzero/variant.h