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>
9 files changed