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

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>

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