Add BMCBootReason Redfish message

In order to demarcate separate FW boots within a continuous stream of
Redfish event logs, add a BMCBootReason message which is logged during
every BMC FW bootup. This message also provides the current FW version
string, as well as a reason describing why the boot happened (e.g. power
loss, user request, watchdog timeout).

Tested: Inserted message into journal using logger-systemd and observed
Redfish event at redfish/v1/Systems/system/LogServices/EventLog/Entries.

Change-Id: I0f0a10ebf300e1a9f2e18bb6735d95e96aeb8473
Signed-off-by: Jonathan Doman <jonathan.doman@intel.com>