Revisit the cpuType type definition

[AkihiroSuda]

I already merged this PR, but on second thought we could define this as []struct{Type string, Arch Arch} so that a single YAML can be used on multiple archs?

Originally posted by @AkihiroSuda in #643 (comment)

[jandubois]

I don't know. Initially I thought that it would rarely be useful, but I can see it for defaults.yaml, so you can override the cpu type defaults for the different architectures.

If we want to change it, we should do it before the next release, so there is no backwards compatibility issue!

[jandubois]

we could define this as []struct{Type string, Arch Arch} so that a single YAML can be used on multiple archs?

I'm going to work on this, but I think it should be a map[Arch]string instead of a slice. It makes no sense to specify multiple CPU types for the same arch. The defaults would be:

cpuType:
  aarch64: cortex-a72
  x86_64: qemu64

Let me know if I misunderstand!

[AkihiroSuda]

map[Arch]string

I thought it is inconsistent with the images struct, but not a huge deal probably, so SGTM

[jandubois]

I thought it is inconsistent with the images struct, but not a huge deal probably, so SGTM

For images it (used to) make sense because you could have a faster local location, and a slower download location for the same arch as a fallback. Now that we have built-in caching, this seems questionable, but probably not worth changing.