ubiformat: open flash image readonly ...

ubiformat only needs readonly access to the image to flash, so
open it readonly. Opening r/w may fail for example on tftp servers.

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

fixup! commands: add ubiformat ...

This adds the missing ubiformat.c file missing from:

commit 5a29d4a
Author: Wolfram Sang <w.sang@pengutronix.de>
Date:   Mon Dec 17 16:48:33 2012 +0100

    commands: add ubiformat

    Imported from mtd-utils and stripped down to needed functionality.
    Based on an older version (1.4.5.) since the newer do use MEMWRITE
    interfaces which we don't have in barebox (yet).

    Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
    Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

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