be tolerant when <boxtyp>_system does not exist

install.py

@@ -356,6 +356,8 @@ def create_if(name, cfg):
 
 def walk(action):
     for rootpath in TO_SYSTEM:
+        if not exists(rootpath):
+            continue
         os.chdir(rootpath)
         for dirpath, _, files in os.walk('.'):
             syspath = dirpath[1:]  # remove leading '.'