realview: fix zImage-initramfs installation

We were copying the actual vmlinux-initramfs.elf kernel which cannot be
launched by QEMU or a real bootloader, use zImage-initramfs instead.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39756 3c298f89-4303-0410-b956-a3cf2f4a3e73