We already have a global_autoboot_state variable that controls barebox
init behavior on startup:

* ABORT      abort and fall into shell
* MENU       display boot menu
* BOOT       boot immediately, only abortable via ctrl+c
             during init
* COUNTDOWN  regular boot after count down

Exporting this as a device parameter allows us to support some
different boot scenarios:

* COUNTDOWN is the default
* ABORT boot always while debugging
* display MENU by default (e.g. for graphical boots)
* BOOT while ignoring external code calling console_countdown_abort()

Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>