sdbus::asio: service sd_event loop upon request

If requested, add in an asynchronous sd_event servicing mechanism so
sd_events can be used in conjunction with boost::asio events. Code
throughout the openbmc repositories use sd_events, especially for
timers. In some cases, we may want to add boost::asio event handling for
superior asynchronous eventing, but not want to rewrite all the existing
sd_event code. This gives us the best of both worlds, with a
low-overhead mechanism to handle the sd_events.

Change-Id: I3f8e2aafa3f733439e1494253698d17c2f3a2321
Signed-off-by: Vernon Mauery <vernon.mauery@linux.intel.com>
diff --git a/Makefile.am b/Makefile.am
index ab07920..ec552c3 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -8,6 +8,7 @@
 	sdbusplus/asio/connection.hpp \
 	sdbusplus/asio/object_server.hpp \
 	sdbusplus/asio/detail/async_send_handler.hpp \
+	sdbusplus/asio/sd_event.hpp \
 	sdbusplus/bus.hpp \
 	sdbusplus/bus/match.hpp \
 	sdbusplus/sdbus.hpp \