07-uart code refactor #460

Job	Run time
build-book	32s
build-f3discovery-doc	1m 9s
build-f3discovery-chapter (05-led-roulette)	32s
build-f3discovery-chapter (06-hello-world)	27s
build-microbit-chapter (05-led-roulette)	29s
build-f3discovery-chapter (07-registers)	30s
build-microbit-doc	2m 8s
build-microbit-chapter (07-uart)	23s
build-f3discovery-chapter (08-leds-again)	29s
build-microbit-chapter (08-i2c)	27s
build-f3discovery-chapter (09-clocks-and-timers)	30s
build-microbit-chapter (09-led-compass)	26s
build-f3discovery-chapter (11-usart)	34s
build-f3discovery-chapter (14-i2c)	31s
build-microbit-chapter (10-punch-o-meter)	37s
build-f3discovery-chapter (15-led-compass)	30s
build-f3discovery-chapter (16-punch-o-meter)	30s
	10m 44s

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