Why?

 * read-only root

 * /etc is pretty bad place for caches

 * all is usually cached by udev in /dev/disk/by-* and libblkid
   is able to use these symlinks

 * boot persistent cache is attractive for very small subset of
   Linux machines (and they already need extra udev tunning otherwise
   udev will probe all block devices during boot)

 * the default is possible to override in /etc/blkid.conf

The systems without /run directory will not be affected by this
change.

Signed-off-by: Karel Zak <kzak@redhat.com>