image: ensure that /dev/console exist in rootfs images

Linux expects that the /dev/console node is present in
the rootfs image. Create the node in rootfs images, in
order to make std{in,out,err} usable even in early init
process.

Note: tar.gz and cpio.gz images are not handled yet.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 37847