common: allow disabling of timeout for password entry

In case a user has to enter a complicated password it is sometimes
desireable to give the user more time than the default timeout.
Enabling this feature will disable the timeout entirely in case the user
presses the <Enter> key before entering any other character.

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>
Reviewed-by: Simon Glass <sjg@chromium.org>