kbuild: rpm-pkg: use kernel-install as a fallback for new-kernel-pkg

The new-kernel-pkg script is only present when grubby is installed, but it
may not always be the case. So if the script isn't present, attempt to use
the kernel-install script as a fallback instead.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>