Add support for the Urboot bootloader #141

Job	Run time
build (ubuntu-latest, examples/arduino-blink)	1m 56s
build (ubuntu-latest, examples/arduino-external-libs)	46s
build (ubuntu-latest, examples/arduino-internal-libs)	34s
build (ubuntu-latest, examples/arduino-own-src_dir)	23s
build (ubuntu-latest, examples/assembly-blink)	23s
build (ubuntu-latest, examples/digitstump-mouse)	28s
build (ubuntu-latest, examples/engduino-magnetometer)	28s
build (ubuntu-latest, examples/native-blink)	27s
build (windows-latest, examples/arduino-blink)	11m 15s
build (windows-latest, examples/arduino-external-libs)	1m 20s
build (windows-latest, examples/arduino-internal-libs)	1m 8s
build (windows-latest, examples/arduino-own-src_dir)	56s
build (windows-latest, examples/assembly-blink)	1m 17s
build (windows-latest, examples/digitstump-mouse)	1m 5s
build (windows-latest, examples/engduino-magnetometer)	1m 14s
build (windows-latest, examples/native-blink)	1m 32s
build (macos-latest, examples/arduino-blink)	3m 28s
build (macos-latest, examples/arduino-external-libs)	56s
build (macos-latest, examples/arduino-internal-libs)	41s
build (macos-latest, examples/arduino-own-src_dir)	56s
build (macos-latest, examples/assembly-blink)	41s
build (macos-latest, examples/digitstump-mouse)	34s
build (macos-latest, examples/engduino-magnetometer)	39s
build (macos-latest, examples/native-blink)	37s
	33m 44s

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