Use the usart-common base plus the usart-v2 code, instead of private
implementations. Less code, more common apis across targets.
Of note is the trick to make F0 look like it has an APB2 bus. It's the
only stm32 that doesn't have a documented APB2 bus, but still has
peripherals enabled via an "APB2" register, and they match how other
targets have an APB2. Simply make APB2 an alias of APB1, as it's only
used for clock speed detection.
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