Upgrade signing key generation

[kaczmarczyck]

This PR adds the upgrade signing key, including:

• generate in setup.sh
• encoding in build.rs
• import in ctap/key_material.rs
• mention in install.md

On top, it adds one missing function to get the partition offset to the interface, and cleans up some unnecessary code after the compiler version jump.
The key is currently unused in key_material.rs. For the bigger picture, please check my upgrade branch.

[coveralls]

Coverage increased (+0.6%) to 42.458% when pulling 664bad1 on kaczmarczyck:submit-upgrade into 7a975ac on google:develop.

[ia0]

I also took a quick look at the upgrade branch. I guess you split the partition location in 2 because of the MPU? I actually realized I could probably do the same for the storage (and avoid the MPU hack :D). I'll try next time I have time.

[ia0]

Last time, after discussion, I realized that #[warn(dead_code)] (and not #[allow(dead_code)]) might be more convenient (no need to change names). As you want.

[kaczmarczyck]

I tried that, and realized that we error on warning. So it's either allow or renaming. And in this case, it wasn't too much work to rename.

[ia0]

Oh right this is not great. We might want to remove that rule in develop branch.

[kaczmarczyck]

@ia0 Yes, the split is to satisfy the MPU. The ugly regex script will make room for new boards eventually though. :)