dfu: mmc: Provide support for eMMC boot partition access

dfu: mmc: Provide support for eMMC boot partition access

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

It sometimes happens that it is necessary to write to other partitions.
This patch adds extra attribute to "raw" sub type of the dfu_alt_info
environment variable (e.g. boot-mmc.bin raw 0x0 0x200 mmcpart 1;)

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

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