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
Commit from Coudu -> https://github.com/CyanogenMod/android_device_sony_montblanc-common/commit/0f58b0b0fba8abd5c4cee0cf8e48ca866d3731ee