d850: update fstab

* add recoveryonly to boot and recovery partitions
* update deprecated prefix on usb/externalsd
* set auto for usb/externalsd filesystem

Change-Id: I7cae47138f249ec33cdba811ba23ae13a67d0b30
Merge branch 'cm-12.0' of https://github.com/CyanogenMod/android_device_lge_d850 into lollipop