env: make env drivers propagate env_import return value

For multiple env drivers to correctly implement fallback when
one environment fails to load (e.g. crc error), the return value
of env_import has to be propagated by all env driver's load
function.

Without this change, the first driver that succeeds to load an
environment with an invalid CRC return 0 (success) and no other
drivers are checked.

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