Add support for the Urboot bootloader #143

Job	Run time
build (ubuntu-latest, examples/arduino-blink)	1m 51s
build (ubuntu-latest, examples/arduino-external-libs)	51s
build (ubuntu-latest, examples/arduino-internal-libs)	48s
build (ubuntu-latest, examples/arduino-own-src_dir)	28s
build (ubuntu-latest, examples/assembly-blink)	19s
build (ubuntu-latest, examples/digitstump-mouse)	33s
build (ubuntu-latest, examples/engduino-magnetometer)	28s
build (ubuntu-latest, examples/native-blink)	20s
build (windows-latest, examples/arduino-blink)	11m 55s
build (windows-latest, examples/arduino-external-libs)	1m 20s
build (windows-latest, examples/arduino-internal-libs)	2m 15s
build (windows-latest, examples/arduino-own-src_dir)	56s
build (windows-latest, examples/assembly-blink)	54s
build (windows-latest, examples/digitstump-mouse)	1m 15s
build (windows-latest, examples/engduino-magnetometer)	1m 12s
build (windows-latest, examples/native-blink)	1m 4s
build (macos-latest, examples/arduino-blink)	3m 23s
build (macos-latest, examples/arduino-external-libs)	1m 13s
build (macos-latest, examples/arduino-internal-libs)	58s
build (macos-latest, examples/arduino-own-src_dir)	48s
build (macos-latest, examples/assembly-blink)	59s
build (macos-latest, examples/digitstump-mouse)	36s
build (macos-latest, examples/engduino-magnetometer)	44s
build (macos-latest, examples/native-blink)	34s
	35m 44s

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