cachedpath.py: global name 'error' is not defined

The error is not global since we don't use "from os import *", so it
should be os.error.

[YOCTO #4489]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
bitbake.conf: define empty EXTRA_IMAGE_FEATURES before using them

* fixes build without EXTRA_IMAGE_FEATURES defined:
  ERROR: '${EXTRA_IMAGE_FEATURES}' in IMAGE_FEATURES is not a valid
  image feature. Valid features: dbg-pkgs ....

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>