gcp-setup: Fixes bug12763

gcp-setup: Fixes bug12763

- This ensures that all ip route and ip rule commands are redirected to null if the output
   is not used to feed into a variable.
- This will prevent any error messages related to empty iproute tables being displayed
   during boot if an empty table is accessed.

Fixes: Bug#12763
Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>