Add MTK6575/6577 EMMC partitions backup support

taken from https://github.com/mrjavum/Team-Win-Recovery-Project/commit/140ff0cde8ad2236caf7bcd480d257d74d8b85bc

"sorocean.gor@gmail.com"

Change-Id: I1a97160c73c7a0bab0535e938d6b35dc09585037