fs/cpio: make cpio rootfs reproducible

fs/cpio: make cpio rootfs reproducible

Pass the recommended argument in the CPIO manual to make cpio archives
reproducible.

Reference: https://www.gnu.org/software/cpio/manual/cpio.html#Copy_002dpass-mode

Pre-patch diffoscope output: https://gitlab.com/snippets/1874745
Post-patch: https://gitlab.com/snippets/1874746

We can see that post-patch, the archive related differences are removed.
The differences are arising from utils/bin/getconf. This will have to
be investigated further. However, that is unrelated to cpio.

Signed-off-by: Atharva Lele <itsatharva@gmail.com>
Reviewed-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>