Fixed mbxxx platform

This is the fixed mbxxx platform.
This mbxxx is completely retro-compatible with actual mb boards, making the mb851 directory almost useless.

Now you can build the examples with
make TARGET=mbxxx CPUREV=CC
or
make TARGET=mbxxx CPUREV=xB
depending on your cpu chip.

e.g.
MB851RevD board has a cpu with
stm32w 108CCU7
so CC is your CPUREV value.

or
MB851RevC board with a cpu
stm32w 108CBU6
so xB is your CPUREV value.
(if the last word is B you can use x as wildcard)

This commit comes from STMicroelectronics staff. We are just cherry-picking their patch due of our collaboration (Istituto Superiore Mario Boella (ISMB) PerT Lab - STM).