Report non-existent file references in config files in more concise

manner.
Fix path finding regular expression to properly catch comma-delimited
paths.

scripts/update_cfg_file.py

@@ -29,7 +29,7 @@ def dont_require_tweaking(fname, content, match_start, match_end):
         (4 <= match_start and # Don't write an arg of 'init=' param.
          content[match_start-4:match_start+1] == 'init=')
 
-def fix_abspath_r(pattern, string, install_dir, iso_name, config_fname):
+def fix_abspath_r(pattern, string, install_dir, iso_name, kept_paths):
     """Return a list of tuples consisting of 'string' with replaced path and a bool representing if /boot/ was prepended in the expression."""
     m = pattern.search(string)
     if not m:
@@ -41,7 +41,7 @@ def fix_abspath_r(pattern, string, install_dir, iso_name, config_fname):
         return [(string[:start] + prologue + '/' + specified_path,
                  '/%s is kept as is.' % specified_path)] \
                 + fix_abspath_r(pattern, string[end:], install_dir, iso_name,
-                                config_fname)
+                                kept_paths)
 
     # See if a path that has 'boot/' prepended is a better choice.
     # E.g. Debian debian-live-9.4.0-amd64-cinnamon has a loopback.cfg
@@ -70,21 +70,29 @@ def fix_abspath_r(pattern, string, install_dir, iso_name, config_fname):
                                 "Removed '.efi' from %s" % specified_path)
     else:
         # Reluctantly accept what is specified.
-        log("Keeping path [%s] in '%s' though it does not exist." % (
-            specified_path, config_fname))
+        if specified_path not in kept_paths:
+            kept_paths.append(specified_path)
         selected_path, fixed = specified_path, False
 
     out = string[:start] + prologue + '/multibootusb/' + iso_name + '/' \
           + selected_path.replace('\\', '/')
     return [(out, fixed)] \
         + fix_abspath_r(pattern, string[end:], install_dir, iso_name,
-                        config_fname)
+                        kept_paths)
 
 def fix_abspath(string, install_dir, iso_name, config_fname):
     """Rewrite what appear to be a path within 'string'. If a file does not exist with specified path, one with '/boot' prepended is tried."""
-    path_expression = re.compile(r'([ \t=])/(.*?)((?=[\s*])|$)')
+
+    path_expression = re.compile(r'([ \t=,])/(.*?)((?=[,|\s*])|$)')
+    kept_paths = []
     chunks = fix_abspath_r(
-        path_expression, string, install_dir, iso_name, config_fname)
+        path_expression, string, install_dir, iso_name,  kept_paths)
+    if len(kept_paths)==1:
+        log("'%s' is kept as is though it does not exist." % kept_paths[0])
+    elif 2<=len(kept_paths):
+        log("Following paths are used as they are though they don't exist.")
+        for kept_path in kept_paths:
+            log('  ' + kept_path)
     tweaked_chunks = [c for c in chunks if c[1]]
     if len(tweaked_chunks) == 0:
         # Fallback to the legacy implementation so that