Motivation: The bluepill uses USART1 as UART0 but other boards like the
STM32 Nucleo boards (and disco as well) use USART2 for USB COM port.
To avoid duplication of code the same pattern as in `machine_atsamd21.go`
is applied where only UART-specific code is moved to `board_*.go`.
The SPI frequency is broken since https://github.com/tinygo-org/tinygo/commit/b8c326d710a88877413e550b06100d9b79373dcd
added I2C interface and changed the unrelated `PCLK2 = HCLK/4` initialization
to `PCLK2 = HCLK/1` (but I2C uses PCLK1 anyways).
This commit changes all baud rate prescalers to be /4 compared to before.
Note: it is not possible to find an equivalent for 125 KHz SPI speed,
it will be too fast (`f = 72 MHz / 256`)
* machine/uart: add core support for multiple UARTs by allowing for multiple RingBuffers
* machine/uart: complete core support for multiple UARTs
* machine/uart: no need to store pointer to UART, better to treat like I2C and SPI
* machine/uart: increase ring buffer size to 128 bytes
* machine/uart: improve godocs comments and use comma-ok idiom for buffer Put/Get methods
cn
Ron Evans
Ayke van Laethem
k-brk