Use configure option yaml path to generate elog-errors.hpp

Add a config variable that can be passed via a recipe to
point the parser to a directory where the error yaml files are.
Rename elog-gen.hpp to elog-errors.hpp, which better matches the
exception error log file names.
Include elog-gen.hpp in elog.hpp so that callers that need to
create and commit error logs only need to include one header file.

Change-Id: Ie3cd65ac761c1f7b2c99c50a2273859283a5ab4b
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/.gitignore b/.gitignore
index 122f6eb..73be2c9 100644
--- a/.gitignore
+++ b/.gitignore
@@ -7,7 +7,7 @@
 compile
 config.*
 depcomp
-elog-gen.hpp
+phosphor-logging/elog-errors.hpp
 install-sh
 logging-test
 logging_test.o