env: sf: use env_import_redund to simplify env_sf_load

For the redundant environment configuration, env_sf_load still
contained duplicate code instead of using env_import_redund().

Simplify the code by only executing the load twice and delegating
everything else to env_import_redund.

Signed-off-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>