Adding UART/USB bootloader project

bootloaders/zero/drivers/cdc_enumerate.c

bootloaders/zero/drivers/cdc_enumerate.h

bootloaders/zero/drivers/uart_driver.c

bootloaders/zero/drivers/uart_driver.h

bootloaders/zero/main.c

bootloaders/zero/main.h

bootloaders/zero/readme.txt

bootloaders/zero/sam_ba_monitor.c

bootloaders/zero/sam_ba_monitor.h

bootloaders/zero/samd21_sam_ba.dep

bootloaders/zero/samd21_sam_ba.ewd

bootloaders/zero/samd21_sam_ba.ewp

bootloaders/zero/samd21_sam_ba.eww

bootloaders/zero/samd21_sam_ba.icf

bootloaders/zero/startup_samd21.c

bootloaders/zero/usart_sam_ba.c

bootloaders/zero/usart_sam_ba.h

bootloaders/zero/utils/compiler.h

bootloaders/zero/utils/interrupt.h

bootloaders/zero/utils/interrupt/interrupt_sam_nvic.c