buildroot: copy KENRNELNAME files to $(KDIR)

In order to support building both normal and initramfs kernel, we will
rebuild the wrappers around the vmlinux ELF binary, copy these with an
appropriate extension when they exist.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37048 3c298f89-4303-0410-b956-a3cf2f4a3e73