Rom table in some devices (e.g. STM32L0/F7) can not be read while
device is in WFI. The Cortex-M SWD signature is however available.
If we know by that signature, that we have a Cortex-M, force a
probe for Cortex-M devices.
The way the const pointer was written caused at least the clang compiler
to complain about double cosnst. I am not sure if the way it was written
before it resulted in the intended "make everything const" goal. But the
way it is written now it adheres to the right to left reading rule.
