bootdeploy: fallback to vmlinuz* kernels when zboot is set
Some kernel packages (e.g. linux-lts in Alpine) don't ship linux.efi, so this needs to fallback to "vmlinuz" or else it won't be able to find a kernel for boot-deploy.
Some kernel packages (e.g. linux-lts in Alpine) don't ship linux.efi, so this needs to fallback to "vmlinuz" or else it won't be able to find a kernel for boot-deploy.