Add support for callbacks

Callbacks are the response in the PDM 'trigger->response' model.
Add general support for implementing callbacks and implement
a log to systemd journal using that framework.

Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
Change-Id: I8bead5368ee5472a02b47e8bba9e9df3a1f346bc
diff --git a/src/templates/journal.mako.cpp b/src/templates/journal.mako.cpp
new file mode 100644
index 0000000..787337d
--- /dev/null
+++ b/src/templates/journal.mako.cpp
@@ -0,0 +1,3 @@
+std::make_unique<Journal<${c.datatype}, phosphor::logging::level::${c.severity}>>(
+${indent(1)}"${c.message}",
+${indent(1)}ConfigPropertyIndicies::get()[${c.instances}])\