Add support for elog callback

Added support for "callback: elog" in PDM.
templates/elog.mako.cpp is used to generate the Elog call.
templates/errors.mako.hpp is used to generate the errors.hpp
used by generated.hpp to include any error.hpp files like
"xyz/openbmc_project/Common/error.hpp"

Change-Id: Id0ae3aca4282095f58a8939b70e398bec706d7f5
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/.gitignore b/.gitignore
index 23155c5..6500baa 100644
--- a/.gitignore
+++ b/.gitignore
@@ -25,4 +25,5 @@
 stamp-h1
 phosphor-dbus-monitor
 generated.hpp
+errors.hpp
 test-driver