Fixes for LLADDR="00:00:00:00:00:00"

Should address at least: bnc#889287, bnc#898620


Huh? `SetS390Vars` is called from two places, not one, and in both cases it will try to get the default value for LLADDR from the variable @s390_defaults. Uselessly, because the default for LLADDR is still defined in `@SysconfigDefaults` where it was used before the assignment got moved.
To complicate things, that default is the zero value we avoid in another commit.
It does make difference, but it is hard to see among the many changes that obscure this bug/fix.