archlinux.ipxe: Turn on OpenSSL CMS signature varification for root file system

[nl6720]

See https://gitlab.archlinux.org/mkinitcpio/mkinitcpio-archiso/-/merge_requests/24 for details.

After there have been at least three releases with OpenSSL signed netboot artifacts, the GPG-based verification option verify=y can be removed.

/cc @dvzrv @Torxed