Provide ARM iso

[anmazzotti]

Describe the solution you'd like:

It should be possible to download an aarch64 based ISO using the same workflow of the amd64 one. This could imply a new variable, possibly in MachineRegistration spec.elemental.install.architecture with either amd64 (default) or aarch64.

This way the elemental-operator will know how to configure the seedimage-builder to provide a downloadable .iso for the right architecture.

There should be no need to support multi-architectures in one MachineRegistration. It is intended for the end-user to have different MachineRegistration for each different architecture.

The aarch64 ISO should be functionally equivalent to the current amd64 one, so with the registration config injected by default.

ManagedOSImage and ManagedOSVersionChannel should also follow the same logic, so that it will be possible to manage aarch64 upgrades the same way. (If that is not the case already, I am not 100% sure)

Anything else you would like to add:

While most ARM boards do not support booting from ISOs, we need to build this as pre-requisite for #947, so that we have all the scaffolding in place to introduce downloadable raw images (for both supported architectures) later on.

[kkaempf]

Small nitpick:

Not sure if amd64 is a good choice. While it is the architecture string used by Go, all of SLES (and afaik also Linux in general) uses x86_64

[kkaempf]

A (UEFI) boot ISO is already built in OBS