dbus: add dbus activation support

Add support for dbus activation of entity manager.  Enabling dbus
activation consists of a few simple steps:

 - Add an activation configuration in /usr/share/dbus-1/system-services
   that maps the dbus service name to the correct systemd service
 - Add a service alias for the service to be dbus activated.  This
   allows dbus activation to be enabled/disabled independently of
   whether the service is started at boot by systemd[1].

[1]https://dbus.freedesktop.org/doc/dbus-specification.html

Tested: systemctl stop xyz.openbmc_project.EntityManager && busctl tree
    xyz.openbmc_project.EntityManager / works
Change-Id: If7dc7f19a19c0ef59370d899f7f67ad6d11064e0
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/service_files/xyz.openbmc_project.EntityManager.service b/service_files/xyz.openbmc_project.EntityManager.service
index 1132835..99452e2 100644
--- a/service_files/xyz.openbmc_project.EntityManager.service
+++ b/service_files/xyz.openbmc_project.EntityManager.service
@@ -11,3 +11,4 @@
 
 [Install]
 WantedBy=multi-user.target
+Alias=dbus-xyz.openbmc_project.EntityManager.service