There are still a few things that the standard says we Shall do and we
don't, but it Works For Me™.  I haven't implemented anything with
regards to GiveBack support, but that doesn't matter just yet.  Our
handling of VDMs isn't quite right either.  Anyway, it successfully
negotiates with so-called Split PDO power supplies, which is more than I
can say about some commercial products.

The original, unmodified portions of ChibiOS retain their original
copyright notices, for the portions of code to which they still apply.

Disable serial, enable I2C2, set clock source to HSI48.

Based on the RT-STM32F072-DISCOVERY demo from ChibiOS/demos/STM32.  The
board definition was made with the Eclipse plugin, and is based on the
ST_STM32F072B_DISCOVERY board definition from ChibiOS/os/hal/boards.