For layerscape images, barebox currently generates a second stage
barebox that lacks the RCW and PBI and as such can be bootm'd as
any other barebox ARM image. A previous commit implemented bootm
handlers for the RCW and PBI prefixed images and thus the 2nd.image
no longer serves a real purpose. Drop it.

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