When using `image-builder upload --to=aws` we do not know
the bootmode. Ideally we would introspect the image for
the boot mode but that is not trivial right now.
So for now just default to platform.BOOT_HYBRID which
translated to `uefi-preferred` in the AWS API calls
which should offer the widest compatbility and should
fix the issue that aarch64 does not boot currently
when uploaded via ibcli.