include: force a kernel-generated cpio removal and rebuild

Make sure that the kernel rebuilds the initramfs cpio archive file by
deleting it before so we get it re-generated properly.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37125 3c298f89-4303-0410-b956-a3cf2f4a3e73