Try harder to make sure that kernel output is world-readable

https://launchpad.net/~ubuntu-cdimage/+livefs/ubuntu/precise/ubuntu-zh-cn currently fails and is disabled. You can't see the failure in the logs (partly because it's only visible in the launchpad-buildd slave log, and partly because those builds were retried and then cancelled before they failed again), but it's as follows:

2014-06-23 12:39:25+0000 [-] Build log: P: Begin unmounting filesystems...
2014-06-23 12:39:25+0000 [-] Iterating with success flag 0 against stage BUILD_LIVEFS
2014-06-23 12:39:28+0000 [-] unexpected error in processEnded
        Traceback (most recent call last):
          File "/usr/lib/python2.7/dist-packages/twisted/internet/process.py", line 916, in maybeCallProcessEnded
            self.reapProcess()
          File "/usr/lib/python2.7/dist-packages/twisted/internet/process.py", line 312, in reapProcess
            self.processEnded(status)
          File "/usr/lib/python2.7/dist-packages/twisted/internet/_baseprocess.py", line 48, in processEnded
            self.maybeCallProcessEnded()
          File "/usr/lib/python2.7/dist-packages/twisted/internet/process.py", line 918, in maybeCallProcessEnded
            _BaseProcess.maybeCallProcessEnded(self)
        --- <exception caught here> ---
          File "/usr/lib/python2.7/dist-packages/twisted/internet/_baseprocess.py", line 60, in maybeCallProcessEnded
            proto.processEnded(Failure(reason))
          File "/usr/lib/launchpad-buildd/lpbuildd/slave.py", line 93, in processEnded
            self.notify(statusobject.value.exitCode)
          File "/usr/lib/launchpad-buildd/lpbuildd/debian.py", line 128, in iterate
            func(success)
          File "/usr/lib/launchpad-buildd/lpbuildd/livefs.py", line 80, in iterate_BUILD_LIVEFS
            self.gatherResults()
          File "/usr/lib/launchpad-buildd/lpbuildd/livefs.py", line 105, in gatherResults
            self._slave.addWaitingFile(path)
          File "/usr/lib/launchpad-buildd/lpbuildd/slave.py", line 409, in addWaitingFile
            self.waitingfiles[os.path.basename(path)] = self.storeFile(path)
          File "/usr/lib/launchpad-buildd/lpbuildd/slave.py", line 388, in storeFile
            f = open(path)
        exceptions.IOError: [Errno 13] Permission denied: '/home/buildd/build-LIVEFSBUILD-56/chroot-autobuild/build/livecd.ubuntu.kernel'

We apply an explicit chmod in a few other cases, but we're missing some.

For SRU verification, it should be sufficient to confirm that the livefs above can now be built in Launchpad.