Revert mechanism for serial generation for Arduino AVRs

[DocMoebiuz]

In #230 we added a new way for generating Serial numbers for the boards. For the PICO this works fine, because the chips have an individual serial numbers.
For the Arduino family, it doesn't work well... we have a batch of Mega boards that all ended up with the same serial.

We want to:

• use a similar logic as before
• instead of using A0 for random seed, we will be using the "millis()" function which will lead to unique serials
• the meachanism for the raspberry Pico will be untouched