lib.sh: get_ldc_subdir(): add optional platform argument to strip

This is needed by IPC4 - which generally does not use the .ldc file but
that's another debate for later; let's just get the check-sof-logger.sh
test passing again for now.

This problem wasn't found until last week when Fred cleaned up and
simplified the internal Intel deployment script. This part of the logic
has too many fallbacks so one of them was probably hiding the issue,
probably picking up some .ldc file in /etc instead.

```
── sof-ipc4
│   ├── arl
│   │   └── community
│   │       └── sof-arl.ri -> ../../mtl/community/sof-mtl.ri
│   ├── arl-s
│   │   └── community
│   │       └── sof-arl-s.ri -> ../../mtl/community/sof-mtl.ri
│   ├── mtl
│   │   └── community
│   │       └── sof-mtl.ri
│   ├── sof-arl.ldc -> sof-mtl.ldc
│   ├── sof-arl-s.ldc -> sof-mtl.ldc
│   └── sof-mtl.ldc
```

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

Author: marc-hb

case-lib/lib.sh

@@ -305,8 +305,10 @@ fake_kern_error()
 
 }
 
+# $1: optional (platform) string to strip. Useful for IPC4.
 get_ldc_subdir()
 {
+    local strip_arg="$1"
     local subdir='intel/sof' # default
     local fw_path
     local fw_path_info='/sys/kernel/debug/sof/fw_profile/fw_path'
@@ -323,6 +325,7 @@ get_ldc_subdir()
             subdir=${subdir%/community}
             subdir=${subdir%/intel-signed}
             subdir=${subdir%/dbgkey}
+            test -z "$strip_arg" || subdir=${subdir%/"$strip_arg"}
         fi
     fi
     printf '%s' "$subdir"
@@ -345,7 +348,7 @@ find_ldc_file()
         }
         ldcFile=/etc/sof/sof-"$platf".ldc
         [ -e "$ldcFile" ] || {
-            local subdir; subdir=$(get_ldc_subdir)
+            local subdir; subdir=$(get_ldc_subdir "$platf")
             ldcFile=/lib/firmware/"${subdir}"/sof-"$platf".ldc
         }
     fi