fs: add uimage support w/ example for cpio

Adds uimage support for various root filesystems.  Uimage support is needed when
using the bootm command in uboot.  This provides both a simple infrastructure
similar to the current BR2_TARGET_ROOTFS_$(2)_GZ for adding compression. To
define the a rootfs should have a UIMAGE wrapper simply create the
BR2_TARGET_ROOTFS_$(2)_UIMAGE symbol.
zedboard: add Avnet Zedboard support.