[2/2] cm: set cache partition location

* In init.rc, this prop will be used to set DOWNLOAD_CACHE
* If a device specifies having a dedicated /cache partition, by setting
  BOARD_CACHEIMAGE_FILE_SYSTEM_TYPE, use it!
* Default to /data/cache per AOSP

Change-Id: I838672ca4638c9665333c1a67257fc435f33f388