make boot stack size explicit

[daef]

just a small cosmetic change

[Woazboat]

You also need to change the actual boot stack definition to use the new constant: https://github.com/IAIK/sweb/blob/main/common/source/kernel/main.cpp#L31

That code needs to be architecture independent, so you need to define the constant (and ideally a forward declaration of the array) in a header file where it's accessible for all architectures.