kbuild: handle compressed cpio initramfs-es

Make kbuild handle compressed cpio initramfs-es.  An already compressed
cpio is copied directly to usr/, while a non-compressed cpio is filtered
through gzip (no changes here) on its way to usr/.

If the user has created a compressed cpio by other means, this saves him
from uncompressing it, just to be compressed again by kbuild.

Signed-off-by: Alex Landau <landau.alex@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>