feat(vm_image_util): Add /usr/.noupdate flag file in PXE images.

This can be used by update_engine as a quick test to determine if it is
running on a system that it can handle. This avoids needing something
like the 'coreos.diskless' kernel command line flag.
Merge pull request #209 from marineam/pxe

feat(vm_image_util): Add /usr/.noupdate flag file in PXE images.