init: don't start hotplug2 if it doesn't exist

The current init script always tries to start hotplug 2, even if
it's not present, so we get an error on boot.

This change checks for the presence of the hotplug2 binary before
trying to start it.

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

SVN-Revision: 9606