sysbuild: ensure warning is only printed when VAR is not used

When calling `sysbuild_get(<out-var> VAR <lookup-var> ...)` user
specifically specify the output variable and in such cases the overwrite
warning should not be printed, because such invocation is similar to a
regular `set(<var> <val>)` CMake call.

Only the invocation where the same variable name is used for lookup and
return variable, then the warning must be printed to avoid unintentional
overwriting of existing variables.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>