Currently bitbake uses time stamp to judge whether a cache file is valid. This is problematic if dynamically setting configuration variables from GUI frontend. It is better to implement a hash value based judging mechanism to validate/invalidate caches.
This bug is solved by hob patches in master tree. http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=8e737db4fc2ab90850c2fe91733011dc4e0a24df http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=a295cdc19f5e96a255d096c37678b197b6e7cda5 http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=927bec5f72230be4f2452d9ef5a747b2c00cb781