copy-firmware: Fix linking directories when using compression

copy-firmware: Fix linking directories when using compression

When `copy-firmware` is called with `--xz` or `--zstd` it will create
broken symlinks for directories:

```
$ ./copy-firmware -v --zstd $dir
[...]
creating link qcom/LENOVO/21BX.zst -> ../sc8280xp/LENOVO/21BX.zst
```

The original target `../sc8280xp/LENOVO/21BX` is a directory. Adding the
compression extension to the directory name breaks the link. The
directory `qcom/sc8280xp/LENOVO/21BX` exists but
`qcom/sc8280xp/LENOVO/21BX.zst` does not exist.

The relative symlink needs to be resolved. If it points to a directory,
create the symlink without the compression extension.

Signed-off-by: Benjamin Drung <benjamin.drung@canonical.com>
Signed-off-by: Josh Boyer <jwboyer@kernel.org>