bti: suppress prompt if fed from a pipe

Do not use readline for non-interactive sessions (e.g. those taking
input from a pipe or file). This way stray prompts are not shown.
Explicit prompts are retained, in case some script expects them.

Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>