Add support for the Urboot bootloader #139

Job	Run time
build (ubuntu-latest, examples/arduino-blink)	1m 36s
build (ubuntu-latest, examples/arduino-external-libs)	47s
build (ubuntu-latest, examples/arduino-internal-libs)	31s
build (ubuntu-latest, examples/arduino-own-src_dir)	36s
build (ubuntu-latest, examples/assembly-blink)	32s
build (ubuntu-latest, examples/digitstump-mouse)	30s
build (ubuntu-latest, examples/engduino-magnetometer)	29s
build (ubuntu-latest, examples/native-blink)	29s
build (windows-latest, examples/arduino-blink)	15m 24s
build (windows-latest, examples/arduino-external-libs)	2m 0s
build (windows-latest, examples/arduino-internal-libs)	1m 41s
build (windows-latest, examples/arduino-own-src_dir)	1m 12s
build (windows-latest, examples/assembly-blink)	1m 4s
build (windows-latest, examples/digitstump-mouse)	1m 5s
build (windows-latest, examples/engduino-magnetometer)	1m 15s
build (windows-latest, examples/native-blink)	56s
build (macos-latest, examples/arduino-blink)	3m 22s
build (macos-latest, examples/arduino-external-libs)	1m 14s
build (macos-latest, examples/arduino-internal-libs)	49s
build (macos-latest, examples/arduino-own-src_dir)	32s
build (macos-latest, examples/assembly-blink)	32s
build (macos-latest, examples/digitstump-mouse)	40s
build (macos-latest, examples/engduino-magnetometer)	1m 0s
build (macos-latest, examples/native-blink)	50s
	39m 6s

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