targets: prepare for supporting normal and initramfs images

In order to support both normal images and initramfs, ensure that each
target sets KERNELNAME properly so that the generic kernel building code
can copy the corresponding files over $(KDIR) with the appropriate
extension. Update the various paths to the kernel and wrapper images
from $(LINUX_DIR)/arch/$(ARCH)/boot/$(foo) to $(KDIR)/$(foo).

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37049 3c298f89-4303-0410-b956-a3cf2f4a3e73