boot: bootutil: add support for multiple hw keys

[jameswalmsley]

• Added support for multiple external keys when using the MCUBOOT_HW_KEY configuration.
• Kept support for the original MCUBOOT_HW_KEY behaviour.

I'd appreciate any feedback so we can get this into a shape that can be merged.

:)

[d3zd3z]

So, there's a tradeoff. Normally, I wouldn't want to add an ifdef to have multiple variants of the same function, but just change the function itself, and make the normal flow without the feature just have a loop of one iteration.

This is, however, a sort of public API.

However, I kind of feel that this is a change worth making, as the fix for implementers will 1. be flagged as a compilation failure, and 2. is easy to fix, accept the argument, and return BOOTUTIL_HW_KEY_NO_MORE if the index is greater than 0. I personal feel the API change is worth avoiding having to add yet more ifdefs to the code.

[jameswalmsley]

So, there's a tradeoff. Normally, I wouldn't want to add an ifdef to have multiple variants of the same function, but just change the function itself, and make the normal flow without the feature just have a loop of one iteration.

This is, however, a sort of public API.

However, I kind of feel that this is a change worth making, as the fix for implementers will 1. be flagged as a compilation failure, and 2. is easy to fix, accept the argument, and return BOOTUTIL_HW_KEY_NO_MORE if the index is greater than 0. I personal feel the API change is worth avoiding having to add yet more ifdefs to the code.

Hi @d3zd3z - yes that was exactly my thinking - that I wanted to preserve the API - and not break anything for current users who have implemented this.

I'm happy with your suggested approach, and I'll update the PR so there's only a single API going forward.

Thanks for the feedback :)

[jameswalmsley]

@d3zd3z I've updated the patch - and I'm testing it on my boards.. I'll confirm that its working.

[jameswalmsley]

@d3zd3z - I can confirm the patch is working on my boards.
Please can you approve the workflows.

[jameswalmsley]

@d3zd3z - I rebased to to fix the CI / test issues.