The documentation says setupmbr should be used with the image named
'barebox', but it should instead be done with the 'barebox.bin'
image. Attempting to use setupmbr on the ELF barebox image simply
fails:

$ ./scripts/setupmbr/setupmbr -s 32 -m ./barebox -d disk.img
No MBR signature found
barebox image seems not valid: Bad MBR signature

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>