dfu: DFU backend implementation

New, separate driver at ./drivers/dfu has been added. It allows platform
and storage independent operation of DFU.
It has been extended to use new MMC level of command abstraction.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Marek Vasut <marex@denx.de>