kbuild/mkspec: fix grub2 installkernel issue

kbuild/mkspec: fix grub2 installkernel issue

mkspec is copying built kernel to temporrary location

	/boot/vmlinuz-$KERNELRELEASE-rpm

and runs installkernel on it. This however directly leads to grub2
menuentry for this suffixed binary being generated as well during the run
of installkernel script.

Later in the process the temporary -rpm suffixed files are removed, and
therefore we end up with spurious (and non-functional) grub2 menu entries
for each installed kernel RPM.

Fix that by using a different temporary name (prefixed by '.'), so that
the binary is not recognized as an actual kernel binary and no menuentry
is created for it.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Fixes: 3c9c7a14b627 ("rpm-pkg: add %post section to create initramfs and grub hooks")
Signed-off-by: Michal Marek <mmarek@suse.com>