This can be enabled via the s6-notify configure option; see
http://www.skarnet.org/software/s6/notifywhenup.html for details.
ndhc will signal that it is ready when the first valid lease is
obtained. Programs dependent on a working network interface
can then simply use s6-wait on the associated ndhc service dir.
A typical command line option assuming the s6 service directory
notification-fd contains '3' would be '--s6-notify 3', and
a typical configure file option would be 's6-notify 3'.