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 <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

scripts/package/mkspec

···
 	%preun
 	if [ -x /sbin/new-kernel-pkg ]; then
 	new-kernel-pkg --remove $KERNELRELEASE --rminitrd --initrdfile=/boot/initramfs-$KERNELRELEASE.img
+	elif [ -x /usr/bin/kernel-install ]; then
+	kernel-install remove $KERNELRELEASE
 	fi
 
 	%postun