kbuild: fix error when building from src rpm

The following issue can be reproduced with Linus' tree on
an x86_64 server.

>+ cp /home/user/rpmbuild-test/BUILDROOT/kernel-3.9.2.x86_64/boot/vmlinuz-3.9.2
>cp: missing destination file operand after
>/home/user/rpmbuild-test/BUILDROOT/kernel-3.9.2-1.x86_64/boot/vmlinuz-3.9.2'
>Try `cp --help' for more information.
>error: Bad exit status from /var/tmp/rpm-tmp.R4o0iI (%install)

Here are the commands to reproduce:

make defconfig
make rpm-pkg

Use the resulting src rpm to build as follows:

mkdir ~/rpmbuild-test
cd ~/rpmbuild-test
rpmbuild --rebuild --define "_topdir `pwd`" -vv ~/rpmbuild/SRPMS/kernel-3.10.0_rc1+-1.src.rpm

The issue is because the %install script uses $KBUILD_IMAGE and it hasn't
been set since it is only available in the kbuild system and not in the
%install script.

This patch adds a Makefile target to emit the image_name that can be used
and modifies the mkspec to use the dynamic name in %install.

Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>