boot -l crashes with CONFIG_MENU disabled because blspec_alloc returns
with blspec->menu being NULL in this case. So guard the usage of
blspec->menu accordingly.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>