[localecfg] Fix mismatch between filenames

Testing for existence of a file in the live system, and then
copying it in the target system, is not a recipe for success.

 - Fix the restore-from-backup part.
 - Document that your live and target system must both have
   /etc/locale.gen if you want this to work at all.

src/modules/localecfg/main.py

@@ -54,11 +54,12 @@ def run():
     target_etc_default_path = "{!s}/etc/default".format(install_path)
 
     # restore backup if available
-    if os.path.exists('/etc/locale.gen.bak'):
-        shutil.copy2(target_locale_gen_bak,
-                     target_locale_gen)
+    if os.path.exists(target_locale_gen_bak):
+        shutil.copy2(target_locale_gen_bak, target_locale_gen)
 
-    # run locale-gen if detected
+    # run locale-gen if detected; this *will* cause an exception
+    # if the live system has locale.gen, but the target does not:
+    # in that case, fix your installation filesystem.
     if os.path.exists('/etc/locale.gen'):
         text = []