Merge branch 'bootloader_rework' of https://github.com/aethaniel/ArduinoCore-samd

CHANGELOG

bootloaders/zero/Makefile

bootloaders/zero/README.md

bootloaders/zero/board_definitions.h

bootloaders/zero/board_driver_led.c

bootloaders/zero/board_driver_led.h

bootloaders/zero/board_driver_serial.c

bootloaders/zero/board_driver_serial.h

bootloaders/zero/board_driver_usb.c

bootloaders/zero/board_driver_usb.h

bootloaders/zero/board_init.c

bootloaders/zero/board_startup.c

bootloaders/zero/bootloader_samd21x18.ld

bootloaders/zero/drivers/cdc_enumerate.c

bootloaders/zero/drivers/cdc_enumerate.h

bootloaders/zero/main.c

bootloaders/zero/readme.txt

bootloaders/zero/sam_ba_cdc.c

bootloaders/zero/sam_ba_cdc.h

bootloaders/zero/sam_ba_monitor.c