SPI: Add read() method for optimization, similar to write().
Add comment about bitfield accessors expecting already shifted values.
Include clock.hpp .
Add comment about CMSIS SystemInit() (called by startup before main).
Factor out led definition, to ease switching to other pins.
Move libopencm3-based stm32 gpio impl to separate file.
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Default STM32 gpio impl now uses CMSIS-like access.