Build - 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: I12cf5aac2c818873a6965db398e7bdbc78326d9d
Signed-off-by: STELIX <ssspinni@gmail.com>