test: Make bootdata accessible in C programs

[CyrilKoe]

This is useful to build crt0 in external SDKs, or external bootroms in SoCs.

[colluca]

What is exactly the use for this? Because it has no use in the snitch_cluster repo, so it's arguable whether it should have a place here or just be defined by whatever external system needs it.

The only benefit I see of hosting it here would be code reuse, is if it's reused by a lot of systems. But the clustergen tool already provides most desired reuse. Not sure we really need to reuse what seems to me as a pretty specific C/C++ file.

Also maybe you want to look at the generated HAL C/C++ headers in the runtime https://github.com/pulp-platform/snitch_cluster/blob/main/target/snitch_cluster/sw/runtime/common/snitch_cluster_cfg.h.tpl. Perhaps this simple interface could also serve your use case. I want to anyways extend it and move it to a more reasonable location.