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.

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.