Implement BmcEpoch set/get elapsed()

Getting elapsed returns BMC's time;
Setting elapsed will check the current time mode/owner, and will return
error when mode is NTP or owner is HOST.

Add unit test cases.

Change-Id: Ibf4e90957f3b26b68c4a1b6dc66dc364c66bef10
Signed-off-by: Lei YU <mine260309@gmail.com>
2 files changed