env_sf: Fix recovery default

The u-boot environment is redundantly stored in a NOR flash on our boards.
Redundant means that there are two places to store the environment. But only
one of the two is active. I discovered that on one board the u-boot (env_sf)
uses the environment from the second place and the Kernel (fw_printenv) uses
the environment from the first place.
To decide which is the active environment there is a byte inside the
environment. 1 means active and 0 means obsolete. But on that board both
environments had have a 1. This can happen if a power loss or reset occurs
during writing the environment. In this situation the u-boot (env_sf)
implementation uses the second environment as default. But the Kernel
(fw_printenv) implementation uses the first environment as default.

This commit corrects the default in the u-boot env_sf implementation when a
problem was detected. Now the recovery default is the same like in all other
environment implementations. E.g. fw_printenv and env_flash. This ensures that
u-boot and Kernel use the same environment.

Signed-off-by: Mario Schuknecht <mario.schuknecht@dresearch-fe.de>