Initial implementation of HostEpoch

When host time is set, the diff between the BmcTime and the value is
saved to persistent storage;
When host time is retrieved, return the BmcTime plus the diff as
host's time.

Add the unit test cases for HostEpoch.

Change-Id: Ia55b93bfcba4f226ceaed8491136ea7afda7bd77
Signed-off-by: Lei YU <mine260309@gmail.com>
diff --git a/bmc_epoch.hpp b/bmc_epoch.hpp
index 56d78e9..8c841a7 100644
--- a/bmc_epoch.hpp
+++ b/bmc_epoch.hpp
@@ -35,5 +35,5 @@
         uint64_t elapsed(uint64_t value) override;
 };
 
-}
-}
+} // namespace time
+} // namespace phosphor