use /run for communication FIFO

Use /run for communication FIFO which is typically preserved
between initramfs and regular root file system. Introduce a
new environment variable PSPLASH_FIFO_DIR which allows to
pass /tmp for the old behavior or another directory.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>