We currently create all boot entries before attempting boot. This is
less than optimal, because this may involve probing devices that won't
be used for actual boot.

In preparation for changing this, refactor the code, so we only have one
loop we need to touch.

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