currently supported only two options: any key and ctrl-c
"any key" is default option.
To configure it, use:
nv autoboot_abort_key=any
or
nv autoboot_abort_key=ctrl-c

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>