Refactor BSP to match more modern Arduino.cc style

[terrillmoore]

The Arduino Portenta BSP has refined the BSP architecture in a number of ways that are sensible (and clearly easier to analyze). We should adopt as many of those changes as are convenient, while reducing dependencies on the ST BSP code.

• There's an Arduino namespace
• Header file breakdown is more logical
• A lot of obsolete things have been removed

We should consider how we can track this. (The Portenta H7 BSP does pull in the STM32 H7 BSP from CubeMx, which is big and really chews up compile cycles; but that's a different story.)