rootfs-cpio: add U-Boot image support

Adds U-Boot image support for cpio root filesystems. This allows you to
use the bootm command in U-Boot to load the rootfs. It makes it possible to
verify the CRC of the initramfs before booting the kernel.

[Spenser: wrote first version of the patch.]
Signed-off-by: Spenser Gilliland <spenser@gillilanding.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>