km/common: stop u-boot only if someone press <SPACE>

To prevent u-boot to stop accidently e.g. due to line noise on the
serial line, we now use the option CONFIG_AUTOBOOT_KEYED. We choose the
<SPACE> key for this.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Reviewed-by: Heiko Schocher <hs@denx.de>