Fix psplash-systemd failures

To avoid racing on FIFO creation between psplash and psplash-systemd,
enhance the systemd support by adding a call to sd_notify in psplash
after the FIFO is created. That will alow setting the psplash-start
systemd unit type to "notify" to avoid the race.

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>