Add base-address-shift configuration flag

[gephaistos]

Hello!
We are working with the FPGA-based project and unlike with micro-controllers all peripherals are dynamic.
In order to swap different devices (described within several SVD files) in a single board we need to be able to move their addresses.
That's why in this PR we added a flag to shift all base addresses provided in SVD.

[burrbull]

Would not it better to generate some global constant and use it calculating PTR?

[Emilgardis]

I don't see the reasoning for this, why is the base address in the svd not shifted already?

[gephaistos]

The address might depend on other cores.
For example, if we want to add our core to these cores the base address should have one value and in the other case it would require another value.
It would be more convenient to be able to change base address value dynamically instead of changing this value in SVD file each time.

[burrbull]

It would be more convenient to be able to change base address value dynamically instead of changing this value in SVD file each time.

What do you mean under "dynamically" in rust terms?
Is it const BASE_ADDRESS or static mut BASE_ADDRESS. In second case it can lead to significant downgrade in code optimization.

[gephaistos]

I meant to dynamically shift it from SVD file during generation, not on the Rust side. The PR does exactly this functionality.

[thejpster]

It seems like a niche use-case, but the PR is low-overhead and most people can just ignore the configuration option. Perhaps the help text should be something like:

This offset will be added to all base addresses on all peripherals in the SVD file. Useful for soft-cores where the peripheral address range isn't necessarily fixed. Ignore this option if you are not building your own FPGA based soft-cores.

[burrbull]

need rebase