main/postmarketos-mkinitfs: fix incorrect cpio usage

cpio expects a newline-separated list of files from stdin by default,
and you can change that to a NULL-terminated list of files by using the
'-0' switch.

Previously we were passing a NULL-terminated list ('-print0') and using
cpio with the default setting. For some reason busybox cpio works with
that but GNU cpio doesn't like that and generates an initramfs without
any files (as it only gets "a single file").

With this change the initramfs generation works fine with both busybox
cpio and gnu cpio.

Fixes #815 (closed)