ubi: env: fix redundand management

ubi: env: fix redundand management

We check (with a #if defined()) the config ENV_UBI_VOLUME_REDUND
to know if there is a redundant env. But this config is a string
and is always defined with env is in ubi, so we always consider
that a redundand env is used.

To fix this issue, I've added a hidden flag ENV_UBI_IS_VOLUME_REDUND
that is true when ENV_UBI_VOLUME_REDUND is not "". Then, I check
this flag in the code, instead of the string ENV_UBI_VOLUME_REDUND.

hs: fixed typo s/condider/consider

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Heiko Schocher <hs@denx.de>