Add support for the Urboot bootloader #140

Job	Run time
build (ubuntu-latest, examples/arduino-blink)	1m 35s
build (ubuntu-latest, examples/arduino-external-libs)	56s
build (ubuntu-latest, examples/arduino-internal-libs)	45s
build (ubuntu-latest, examples/arduino-own-src_dir)	23s
build (ubuntu-latest, examples/assembly-blink)	19s
build (ubuntu-latest, examples/digitstump-mouse)	22s
build (ubuntu-latest, examples/engduino-magnetometer)	37s
build (ubuntu-latest, examples/native-blink)	21s
build (windows-latest, examples/arduino-blink)	14m 44s
build (windows-latest, examples/arduino-external-libs)	2m 7s
build (windows-latest, examples/arduino-internal-libs)	1m 15s
build (windows-latest, examples/arduino-own-src_dir)	1m 10s
build (windows-latest, examples/assembly-blink)	1m 16s
build (windows-latest, examples/digitstump-mouse)	56s
build (windows-latest, examples/engduino-magnetometer)	1m 20s
build (windows-latest, examples/native-blink)	48s
build (macos-latest, examples/arduino-blink)	2m 38s
build (macos-latest, examples/arduino-external-libs)	1m 5s
build (macos-latest, examples/arduino-internal-libs)	51s
build (macos-latest, examples/arduino-own-src_dir)	41s
build (macos-latest, examples/assembly-blink)	33s
build (macos-latest, examples/digitstump-mouse)	40s
build (macos-latest, examples/engduino-magnetometer)	1m 0s
build (macos-latest, examples/native-blink)	40s
	37m 2s

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