dfu, nand: add medium specific polltimeout function

add a possibility to add a medium specific polltimeout
function. So it is possible to define different
poll timeouts.

Used on nand medium, for setting the DFU_MANIFEST_POLL_TIMEOUT
only on nand ubi partitions, which is currently the only
usecase.

Change-Id: If1db5f49b32d93fefa7481e8dfe5b7ccc0e65af4
Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>