include/image.mk: introduce KERNEL_INITRAMFS_NAME variable

In case if we're interested in generation of non-binary images
(for example if we want images to be valid .elfs - and that's what we
want for ARC now) we have to instruct Device/Build/initramfs target to
use input file with some extension (in case of ARC it will be
essentially .elf).

Otherwise default binary vmlinux-initramfs (without extension) is used and
resulting images then are binaries instead of elfs.

Now having KERNEL_INITRAMFS_NAME variable it's possible to substitute
default $$(KERNEL_NAME)-initramfs with preferred option like
$$(KERNEL_NAME)-initramfs.elf.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Felix Fietkau <nbd@openwrt.org>
Cc: John Crispin <blogic@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@47403 3c298f89-4303-0410-b956-a3cf2f4a3e73