Add support for updating sketches from an SD card

cores/arduino/SERCOM.cpp

extras/package_index.json.Hourly.template

extras/package_index.json.PR.template

libraries/SDU/extras/SDUBoot/SDUBoot.ino

libraries/SDU/extras/SDUBoot/build.sh

libraries/SDU/keywords.txt

libraries/SDU/library.properties

libraries/SDU/src/SDU.cpp

libraries/SDU/src/SDU.h

libraries/SDU/src/boot/mkr1000.h

libraries/SDU/src/boot/mkrzero.h

libraries/SDU/src/boot/zero.h

platform.txt

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

variants/mkr1000/linker_scripts/gcc/flash_with_bootloader.ld

variants/mkrzero/linker_scripts/gcc/flash_with_bootloader.ld