This change adds a common I2C class, gathering various I2C functions into a single point of responsibility.
It's necessary for correctly managing the I2C bus pins and state across multiple devices.
* Add a common/pimoroni.hpp to list default pins for various add-ons
* Move the BG SPI Slot enum here for safe keeping
* Switch all GPIO pin references to "uint" to match Pico SDK and bring back PIN_UNUSED as UINT_MAX
This change removes file-relative include paths and adds the project root as a global include path.
* Project root added to CMakeLists.txt so that all targets can find includes
* Project root added micropython.cmake so that targets used by the MicroPython build can find includes
Note: pico-boilerplate projects must set this include path
Phil Howard