env: mmc: rework mmc_env_partition_by_guid() to work with two separate partitions

env: mmc: rework mmc_env_partition_by_guid() to work with two separate partitions

Having two separate partitions for use in a redundant environment
setup works just fine, if one only relies on setting CONFIG_ENV_OFFSET
and CONFIG_ENV_OFFSET_REDUND. However, if CONFIG_PARTITION_TYPE_GUID
is enabled, the current logic in mmc_env_partition_by_guid() means
that only the first partition will ever be considered, and prior to
the previous commit, lead to silent data corruption.

Extend the logic so that, when we are looking for the location for the
second copy of the environment, we keep track of whether we have
already found one matching partition. If a second match is found,
return that, but also modify *copy so that the logic in the caller
will use the last ENV_SIZE bytes of that second partition - in my
case, and I suppose that would be typical, both partitions have been
created with a size of exactly the desired ENV_SIZE.

When only a single matching partition exists, the behaviour is
unchanged: We return that single partition, and *copy is left as-is,
so the logic in the caller will either use the last (copy==0) or
second-to-last (copy==1) ENV_SIZE bytes.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>