Commit e5098495 "usb: gadget: fastboot: Add option to download to a buffer"
breaks the use of temporary files for fastboot barebox_update.

In the patch the ".imagefile = FASTBOOT_TMPFILE" allocation gets
removed, but data.imagefile is still used. This leads to an NULL
pointer dereference. We fix it by using sourcefile instead.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>