sepolicy: mount firmware partitions with firmware_file context

* we don't need to give ueventd and mdm_helper access across the
  entire vfat context

Change-Id: Ie50be652d40e87f09f8becc8b33aed472ad54d76
Merge branch 'cm-12.0' of https://github.com/CyanogenMod/android_device_samsung_jf-common into cm-12.0