feat(sys-apps/baselayout): Stop changing the core user's UID

The SDK does not set the `symlink-usr` flag so it has stuck with the old
amd64-generic behavior of setting the core user's UID to 1000. In order
to make running the SDK on CoreOS reasonably sane switch to 500 on both.
Merge pull request #639 from marineam/coreception

feat(sys-apps/baselayout): Stop changing the core user's UID