Parameterise STM32 I2C pin modes and timing parameters. (#5671)

I2C timing parameters were seemingly set up for an STM32F303 target MCU, at a specific clock speed. This commit allows specifying the timing parameters via config.h, allowing other STM32 MCUs to be targeted, potentially at different clock frequencies.
Alternate function modes for the I2C pins are now also configurable, allowing for remapping to other pins.