static-nodes: indicate that creation of static nodes should only happen at boot

udev will only manage static nodes that exist at the time udev is started, so
creating static nodes later on will likely not behave as expected. In
particular, recreating the static nodes at run-time will reset any permissions
udev may have applied to the nodes at boot.

See <https://bugzilla.redhat.com/show_bug.cgi?id=1147248> and the discussion
following <http://permalink.gmane.org/gmane.comp.sysutils.systemd.devel/23795>.

Note that this requires (the yet to be released) systemd v217 or a backport
of systemd patch 8c94052ee543c3598a3c7b0c46688150aa2c6168.