dfu, nand, ubi: add partubi alt settings for updating ubi partition

updating an ubi partition needs a completely erased mtd partition,
see:
http://lists.infradead.org/pipermail/linux-mtd/2011-May/035416.html



So, add partubi alt setting for the dfu_alt_info environment
variable to mark this partition as an ubi partition. In case we
update an ubi partition, we erase after flashing the image into the
partition, the remaining sektors.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Tom Rini <trini@ti.com>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Scott Wood <scottwood@freescale.com>