xtensa-build-zephyr.py: show symbolic links at the end

Not sure why I thought it would be complicated; I should have done this
much sooner.

Before:

```
build-sof-staging
|-- sof
|   +-- intel
|       +-- sof-ipc4
|           |-- adl
|           |   +-- community
|           |       +-- sof-adl.ri
|           |-- adl-n
|           |   +-- community
|           |       +-- sof-adl-n.ri
|           |-- arl
|           |   +-- community
|           |       +-- sof-arl.ri

```

After:

```
build-sof-staging
|-- sof
|   +-- intel
|       +-- sof-ipc4
|           |-- adl
|           |   +-- community
|           |       +-- sof-adl.ri  -> ../../tgl/community/sof-tgl.ri
|           |-- adl-n
|           |   +-- community
|           |       +-- sof-adl-n.ri  -> ../../tgl/community/sof-tgl.ri
|           |-- arl
|           |   +-- community
|           |       +-- sof-arl.ri  -> ../../mtl/community/sof-mtl.ri

```

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

scripts/xtensa-build-zephyr.py

@@ -378,13 +378,17 @@ def show_installed_files():
 
 	for pre, _, node in RenderTree(graph_root, render.AsciiStyle):
 		fpath = STAGING_DIR / node.long_name
+
+		# pathLib.readlink() requires Python 3.9
+		symlink_trailer = f" -> {os.readlink(fpath)}" if fpath.is_symlink() else ""
+
 		stem = node.name[:-3] if node.name.endswith(".gz") else node.name
 
 		shasum_trailer = ""
 		if checksum_wanted(stem) and fpath.is_file() and not fpath.is_symlink():
 			shasum_trailer =  "\tsha256=" + checksum(fpath)
 
-		print(f"{pre}{node.name} {shasum_trailer}")
+		print(f"{pre}{node.name} {symlink_trailer} {shasum_trailer}")
 
 
 # TODO: among other things in this file it should be less SOF-specific;