There are two differences between file_write_flash() and write_image()
(which is removed in this patch):

 - file_write_flash() uses write_full() vs. write() in write_image()
 - file_write_flash() doesn't erase the whole device but only the needed
   space

So the new code should be more robust and maybe even faster.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>