Add support for CM7 / STM32H7 family processors

[terrillmoore]

On an upcoming USB project, we're using an STM32H processor, which is a Cortex M7. Opening this ticket to track modifications for supporting the STM32H7 family (and incidentally for the Cortex M7 / "cm7" architecture).

These processors have lots of on-chip flash relative to our needs, so we'll support staging images to on-chip flash as well as to external devices.