[U-Boot] cmd_nvedit.c: Add env exists command

[Tom Rini]

On Tue, Oct 22, 2013 at 07:07:34PM -0500, Andrew Ruder wrote:

> env exists is a way to test (in hush) if an environment variable
> exists.  A workaround existed using printenv but this new command
> doesn't require all the stdout/stderr redirection to prevent
> printing information to the screen.
> 
> Example:
> $ set testexists 1
> $ env exists testexists && echo "yes"
> yes
> $ env exists testexists || echo "no"
> $ set testexists
> $ env exists testexists && echo "yes"
> $ env exists testexists || echo "no"
> no
> $
> 
> Signed-off-by: Andrew Ruder <andrew.ruder at elecsyscorp.com>

Applied to u-boot/master, thanks!

-- 
Tom
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