At the moment e.g. a wait for an ARP response during net boot can only
aborted if the auto boot countdown was aborted. Otherwise ctrl+c is
without effect. For better user experience allow code querying for ctrl+c
to see it even if barebox had not dropped into a shell.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>