07-uart code refactor #474

Job	Run time
build-f3discovery-doc	1m 19s
build-book	21s
build-f3discovery-chapter (05-led-roulette)	26s
build-microbit-chapter (05-led-roulette)	36s
build-f3discovery-chapter (06-hello-world)	31s
build-microbit-doc	2m 22s
build-microbit-chapter (07-uart)	40s
build-f3discovery-chapter (07-registers)	32s
build-microbit-chapter (08-i2c)	47s
build-f3discovery-chapter (08-leds-again)	25s
build-microbit-chapter (09-led-compass)	41s
build-f3discovery-chapter (09-clocks-and-timers)	25s
build-microbit-chapter (10-punch-o-meter)	33s
build-f3discovery-chapter (11-usart)	27s
build-f3discovery-chapter (14-i2c)	43s
build-f3discovery-chapter (15-led-compass)	28s
build-f3discovery-chapter (16-punch-o-meter)	28s
	11m 44s

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