fs/initramfs: refactor with fs/cpio

An initramfs is in fact the same as a cpio archive, but embedded in
the kernel.  So instead of duplicating the cpio infrastructure,
we can simply build images/rootfs.cpio and link that into the
kernel.

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