The ubiformat C API expects an image file as argument. With upcoming
Android fastboot sparse image support we can no longer provide a
complete image anymore. With this patch we can write an image in
multiple chunks after we've formatted a MTD device with ubiformat.

ubiformat_write will skip the EC header in both the MTD device we
write to and also the image we read from, so we can flash an image
on a MTD device containing EC headers already.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>