[WIP] Update the microbit part of the book #448

Job	Run time
build-f3discovery-doc	1m 4s
build-book	17s
build-microbit-chapter (05-led-roulette)	8s
build-f3discovery-chapter (05-led-roulette)	38s
build-microbit-chapter (07-uart)	8s
build-microbit-doc	8s
build-f3discovery-chapter (06-hello-world)	23s
build-microbit-chapter (08-i2c)	5s
build-f3discovery-chapter (07-registers)	29s
build-microbit-chapter (09-led-compass)	11s
build-f3discovery-chapter (08-leds-again)	25s
build-microbit-chapter (10-punch-o-meter)	9s
build-f3discovery-chapter (09-clocks-and-timers)	24s
build-f3discovery-chapter (11-usart)	31s
build-f3discovery-chapter (14-i2c)	25s
build-f3discovery-chapter (15-led-compass)	26s
build-f3discovery-chapter (16-punch-o-meter)	27s
	6m 18s

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