Add support for the Urboot bootloader #142

Job	Run time
build (ubuntu-latest, examples/arduino-blink)	2m 18s
build (ubuntu-latest, examples/arduino-external-libs)	50s
build (ubuntu-latest, examples/arduino-internal-libs)	41s
build (ubuntu-latest, examples/arduino-own-src_dir)	30s
build (ubuntu-latest, examples/assembly-blink)	29s
build (ubuntu-latest, examples/digitstump-mouse)	23s
build (ubuntu-latest, examples/engduino-magnetometer)	28s
build (ubuntu-latest, examples/native-blink)	23s
build (windows-latest, examples/arduino-blink)	9m 48s
build (windows-latest, examples/arduino-external-libs)	1m 40s
build (windows-latest, examples/arduino-internal-libs)	1m 19s
build (windows-latest, examples/arduino-own-src_dir)	1m 5s
build (windows-latest, examples/assembly-blink)	1m 7s
build (windows-latest, examples/digitstump-mouse)	1m 22s
build (windows-latest, examples/engduino-magnetometer)	1m 18s
build (windows-latest, examples/native-blink)	1m 6s
build (macos-latest, examples/arduino-blink)	2m 42s
build (macos-latest, examples/arduino-external-libs)	1m 15s
build (macos-latest, examples/arduino-internal-libs)	52s
build (macos-latest, examples/arduino-own-src_dir)	37s
build (macos-latest, examples/assembly-blink)	40s
build (macos-latest, examples/digitstump-mouse)	34s
build (macos-latest, examples/engduino-magnetometer)	38s
build (macos-latest, examples/native-blink)	32s
	32m 37s

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