entity-manager: add support for dbus activation

Add the dependency on dbus-1, which EM needs for some path variables in
its pkg-config file, and update to handle the additional dbus activation
unit file artifact.

(From meta-phosphor rev: 2a141b6951e19f67d94f7f1983c0115a909aef56)

Change-Id: I982635578036c2c8219c0446213e52a80d439e38
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>