kernel-install: drop redundant "/"

Follow-up for 1cdbff1c.

After the commit 1cdbff1c, each entry .conf contains
redundant slash like the following:
```
$ cat xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx-5.9.8-200.fc33.x86_64.conf
title      Fedora 33 (Thirty Three)
version    5.9.8-200.fc33.x86_64
machine-id xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
options    root=/dev/nvme0n1p2 ro rootflags=subvol=system/fedora selinux=0 audit=0
linux      //xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/5.9.8-200.fc33.x86_64/linux
initrd     //xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/5.9.8-200.fc33.x86_64/initrd
```