Merge pull request #570 from nikgul/microbit #437

Job	Run time
build-f3discovery-doc	1m 4s
build-f3discovery-chapter (05-led-roulette)	30s
build-book	1m 53s
build-microbit-doc	2m 4s
build-microbit-chapter (05-led-roulette)	28s
build-f3discovery-chapter (06-hello-world)	28s
build-microbit-chapter (07-uart)	30s
build-f3discovery-chapter (07-registers)	30s
build-microbit-chapter (08-i2c)	35s
build-f3discovery-chapter (08-leds-again)	20s
build-microbit-chapter (09-led-compass)	40s
build-f3discovery-chapter (09-clocks-and-timers)	23s
build-microbit-chapter (10-punch-o-meter)	30s
build-f3discovery-chapter (11-usart)	28s
build-f3discovery-chapter (14-i2c)	20s
build-f3discovery-chapter (15-led-compass)	22s
build-f3discovery-chapter (16-punch-o-meter)	22s
	11m 27s

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