Since images without the extra header now boot correctly, stop adding
it.  This makes the image 512 bytes smaller.

It also gives the image a correct barebox header.  The 512 byte extra
header looked like a barebox header (had a "barebox\n" signature) but
had incorrect size and text start fields.

Signed-off-by: Trent Piepho <tpiepho@kymetacorp.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>