- Make use of the fact that gpio_init_af() does not need prior call to
gpio_init() for all STM32 families anymore and drop call to gpio_init()
- Initialize the UART periph first, before initializing the pins
- While uninitialized, the UART periph will send signal LOW to TXD. This
results in a start bit being picked up by the other side.
- Instead, we do not connect the UART periph to the pins until it is
initialized, so that the TXD level will already be HIGH when the pins
are attached.
- This results in no more garbage being send during initialization
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