sb/path: Walk up to root checking if a path is writable.

A dirname of / is / so the path will never have a length of 0.

Close #3392

source-builder/sb/path.py

@@ -143,7 +143,7 @@ def iswritable(path):
 
 def ispathwritable(path):
     path = shell(path)
-    while len(path) != 0:
+    while len(path) > 1:
         if exists(path):
             return iswritable(path)
         path = dirname(path)