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>

copy-firmware.sh

@@ -94,9 +94,16 @@ grep -E '^Link:' WHENCE | sed -e 's/^Link: *//g;s/-> //g' | while read f d; do
             $verbose "WARNING: missing target for symlink $f"
         fi
     else
-        install -d "$destdir/$(dirname "$f")"
-        $verbose "creating link $f$compext -> $d$compext"
-        ln -s "$d$compext" "$destdir/$f$compext"
+        directory="$destdir/$(dirname "$f")"
+        install -d "$directory"
+        target="$(cd "$directory" && realpath -m -s "$d")"
+        if test -d "$target"; then
+            $verbose "creating link $f -> $d"
+            ln -s "$d" "$destdir/$f"
+        else
+            $verbose "creating link $f$compext -> $d$compext"
+            ln -s "$d$compext" "$destdir/$f$compext"
+        fi
     fi
 done