remove mount/unmount capabilities from backuptool

backuptool should not be messing with whether /system is mounted
or not as it screws up the expectations of other scripts run
in the install process. instead the mounting/unmounting
functionality has been moved to ota_from_target_files

Change-Id: I0711afd517638e7d0a0c39369d3a776748245dd2
backuptool: Disregard known bad backup scripts

Keep a blacklist of md5 sums for scripts known to cause issues, and
ignore them when installing new builds

Change-Id: I19a88b58194a32da5eb5fe278f2c5b9a145b57be