[U-Boot] getenv_yesno() and autostart

Commit 5a442c0addc69d0c4b58e98e5aec1cf07576debb switched NAND
(among other things) from explicitly checking autostart for "yes"
to using getenv_yesno().

Instead of checking for "yes", getenv_yesno() checks for "no", and assumes
"yes" if it can't find anything.  This changes the default behavior, with no
autostart environment variable defined, from "no" to "yes".  I don't think
"yes" is a desireable default for autostart[1].  README does not specify the
default behavior.  It also doesn't specify that autostart applies to NAND
commands at all, for that matter.

Should getenv_yesno()'s behavior be changed?  Are there cases where
a default of "yes" is legitimately wanted (if so, a parameter could
be added for the requested default)?

-Scott

[1] I'm not sure that the feature makes sense at all, as often one would
need to load an RFS and/or a device tree first, not to mention that you may
be trying to update U-Boot itself.  Autostart doesn't do anything that you
couldn't do yourself by following the load command with "bootm $fileaddr".