bitbake-layers: fix error on startup caused by recent cooker change

Fix bitbake-layers "TypeError: 'NoneType' object is not iterable" error on
startup. Commit f3be8e9a7d changed to expect
prefile and postfile to be populated and no longer expects file.

(Bitbake rev: 0f45fcab5c724aabb5b2933dfcdf88ebe256cba9)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/bin/bitbake-layers

@@ -211,7 +211,8 @@ class Config(object):
         self.pkgs_to_build = []
         self.debug_domains = []
         self.extra_assume_provided = []
-        self.file = []
+        self.prefile = []
+        self.postfile = []
         self.debug = 0
         self.__dict__.update(options)