Allow base path overrides when running a recipe.

recipe_modules/path/api.py

Index: recipe_modules/path/api.py
diff --git a/recipe_modules/path/api.py b/recipe_modules/path/api.py
index c771a803bdc46f7aa4e904861c9dab76f4b6781a..8a46f6d1aac5c269b97767109e907875ec4239f0 100644
--- a/recipe_modules/path/api.py
+++ b/recipe_modules/path/api.py
@@ -125,7 +125,7 @@ def _split_path(path):  # pragma: no cover
     abs_path[0] = '/'
   else:
     assert False, 'Got unexpected path format: %r' % abs_path
-  return abs_path
+  return tuple(abs_path)
 
 
 class PathApi(recipe_api.RecipeApi):
@@ -150,6 +150,11 @@ class PathApi(recipe_api.RecipeApi):
     return {
       'CURRENT_WORKING_DIR': self._startup_cwd,
       'TEMP_DIR': self._temp_dir,
+      'BASE_PATH_OVERRIDES': {
+        k: _split_path(v)
+        for k, v in
+            self._engine.properties.get('base_path_overrides', {}).iteritems()
+      },
     }
 
   def __init__(self, **kwargs):