PEL: Add repository to save PELs

Create the Repository class that can save PELs in (and later retrieve
them from) the filesystem.  It provides an add() method that can add
a PEL object to the repository.

Now, when the Manager class sees an OpenBMC event log created with the
RAWPEL metadata in the AdditionalData property that points at a file
that contains a PEL, it can save that PEL.  Before the PEL is saved, the
log ID and commit timestamp fields in the PEL will be updated - the log
ID to a unique value, and the timestamp to the current time.

Change-Id: I8dbaddf0f155bcb6d40b933294ada83feb75ce53
diff --git a/extensions/openpower-pels/openpower-pels.mk b/extensions/openpower-pels/openpower-pels.mk
index 04106fd..def28a1 100644
--- a/extensions/openpower-pels/openpower-pels.mk
+++ b/extensions/openpower-pels/openpower-pels.mk
@@ -6,4 +6,5 @@
 	extensions/openpower-pels/paths.cpp \
 	extensions/openpower-pels/pel.cpp \
 	extensions/openpower-pels/private_header.cpp \
+	extensions/openpower-pels/repository.cpp \
 	extensions/openpower-pels/user_header.cpp