msm-firmware-loader assumes the device uses emmc when looking for partitions

msm-firmware-loader was initially written for old msm8916 devices that were only ever shipped with emmc. Since it's designed to run in an environment where nothing besides busybox exists, the script was written to parse sysfs to discover partition labels, and as a safeguard/optimization the rule to loop over partition assumes mmcblk named devices.

This makes the script useless for ufs based devices.

Cc: @calebccff