update ota_from_target_files to handle mounting/unmounting for backupscript

backupscript should not be mounting/unmounting itself as it makes other
scripts have unexpected results (such as modelid_cfg, which expects /system
to be mounted)

instead have the ota script handle the mounting/unmounting

Change-Id: I94511f4147c624d975cb3ecbeaa8b0e98f63437c