If initramfs is enabled, build a second pass kernel containing the CPIO
rootfs, preliminary work to get non exclusive enabling of initramfs.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37047 3c298f89-4303-0410-b956-a3cf2f4a3e73