cpio: fix boot with dynamic /dev

Devtmpfs doesn't get automounted by the kernel when an initramfs is used.
Since cpio archives are used almost exclusively as an initramfs, the same
fix should be applied to it as for the initramfs.

Cfr. commit 424888e4 and
10a130f9.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>