gen_init_cpio: remove leading `/' from file names

When we extracted the generated cpio archive using "cpio -id" command,
it complained,

cpio: Removing leading `/' from member names
var/run
cpio: Removing leading `/' from member names
var/lib
cpio: Removing leading `/' from member names
var/lib/misc

It is worse with the latest "cpio" or "pax", which tries to overwrite
the host file system with the leading '/'.

So the leading '/' of file names should be removed. This is consistent
with the initramfs come with major distributions such as Fedora or
Debian, etc.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Acked-by: Mike <Frysinger&lt;vapier@gentoo.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>