kconfig: allow stdin/stdout redirection

Allow stdin/stdout redirection for the CLI conf (not mconf).
This allows to recall a sample and automatically apply the defaults
to new configuration option, with something like the following:
yes "" |ct-ng "sample_name"

 reporter_name="YEM"

 reporter_url="http://ymorin.is-a-geek.org/"

 reporter_comment="EXPERIMENTAL Super-H sh4 sample."