discover: add reinit path

discover: add reinit path

Currently, changes to settings doesn't take effect while the discover
server is running. This means we need to reboot for any changes (eg, to
network settings) to take effect.

This change introduces a reinit path. Triggered by a configuration
update, this will cause the device handler to drop all of its devices
(and boot options), and restart the discovery process from the device
sources.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>