The layerscape images are preceeded by a RCW and PBI, which are interpreted
by the Layerscape Hardware Pre-Bootloader and aren't executable as ARM code.
To maintain the ability to network boot them, add a bootm handler that
skips the fixed-size header.

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