[U-Boot] env_nand.c: support falling back to redundant env when writing

[Scott Wood]

On Fri, Jul 19, 2013 at 12:20:26PM +0200, Phil Sutter wrote:
> Without this patch, when the currently chosen environment to be written
> has bad blocks, saveenv fails completely. Instead, when there is
> redundant environment fall back to the other copy. Environment reading
> needs no adjustment, as the fallback logic for incomplete writes applies
> to this case as well.
> 
> Signed-off-by: Phil Sutter <phil.sutter at viprinet.com>

Applied to u-boot-nand-flash

-Scott