initramfs-init: add support for ssh installer

Created by: tmh1999

This commit allows running the Alpine installer through SSH either using a SSH key or a password. Allowing root access through SSH must be revoked before finishing running setup-disk as this config would leak to new system.

On s390x, the boot media consist of a kernel, an initramfs, and a parm file. We don't necessarily need a modloop image and fetch initial packages from remote repo instead. This behavior is the same on other distros.