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>
diff --git a/env/Kconfig b/env/Kconfig
index e4ba12e..bc03816 100644
--- a/env/Kconfig
+++ b/env/Kconfig
@@ -516,6 +516,12 @@
 	help
 	  Name of the redundant volume that you want to store the environment in.
 
+config ENV_UBI_IS_VOLUME_REDUND
+	bool
+	depends on ENV_IS_IN_UBI
+	default y if ENV_UBI_VOLUME_REDUND != ""
+	default n
+
 config ENV_UBI_VID_OFFSET
 	int "ubi environment VID offset"
 	depends on ENV_IS_IN_UBI