MAINLINE dfu: mmc: Provide support for eMMC boot partition access

MAINLINE dfu: mmc: Provide support for eMMC boot partition access

Before this patch it was only possible to access only the default eMMC
partition. By partition selection I mean the access to eMMC via ext_csd[179]
register programming.

It sometimes happens that it is necessary to write to other partitions.
This patch adds extra attributes to "mmc" sub type of the dfu_alt_info
variable (e.g. boot-mmc.bin mmc 0 200 mmcpart 1;)

It saves the original boot value and restores it after storing the file.

Such definition will not impact other definitions of the "mmc".

Change-Id: I34069510eb27aa80794189d2d13cdb97e54ba83d
Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>