montblanc: fix recovery backuptool

  Fixes recovery backuptool error:
  "grep: /system/build.prop: No such file or directory
  Not backing up files from incompatible version."
  because backup script no longer mounts/unmounts /system:
  https://github.com/CyanogenMod/android_vendor_cm/commit/541720b00d008f094b9fb3336df25053eb2d4d11
  https://github.com/CyanogenMod/android_build/commit/5e8baf9070a49e7b1690144229c1b455315e7f2c

Change-Id: I44a60ad37a7a295c82261d79df9136ab187a33d4