07-uart code refactor #459

Job	Run time
build-f3discovery-doc	1m 10s
build-book	4m 1s
build-microbit-chapter (05-led-roulette)	25s
build-f3discovery-chapter (05-led-roulette)	33s
build-microbit-chapter (07-uart)	17s
build-microbit-doc	30s
build-f3discovery-chapter (06-hello-world)	29s
build-microbit-chapter (08-i2c)	24s
build-f3discovery-chapter (07-registers)	29s
build-microbit-chapter (09-led-compass)	24s
build-f3discovery-chapter (08-leds-again)	27s
build-microbit-chapter (10-punch-o-meter)	25s
build-f3discovery-chapter (09-clocks-and-timers)	29s
build-f3discovery-chapter (11-usart)	34s
build-f3discovery-chapter (14-i2c)	31s
build-f3discovery-chapter (15-led-compass)	28s
build-f3discovery-chapter (16-punch-o-meter)	31s
	12m 7s

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