Comment 2 for bug 1373467

I ran into a snag. The API provides for a "last update date" which is currently calculated from mtime of the first of these files:

* /userdata/.last_update
* /etc/system-image/channel.ini
* /etc/ubuntu-build

Of course, if we get rid of the last two, then only the first is relevant, but that file does not exist in the normal test suite. I'm not sure what the right substitute is, perhaps the latest mtime of any file in /etc/system-image/config.d? I'm going with that for now, but we'll have to discuss whether there's something better we can use.