dfu: mmc: Replace calls to u-boot commands with native mmc API

dfu: mmc: Replace calls to u-boot commands with native mmc API

For some time we have been using the run_command() with properly crafted
string. Such approach turned to be unreliable and error prone.

Switch to "native" mmc subsystem API would allow better type checking and
shall improve speed.

Also, it seems that this API is changing less often than u-boot commands.
The approach similar to env operations on the eMMC has been reused.

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