fw-inventory: Provide BiosVersion in system obj

Created a new fw_utils.hpp to reduce more indentation code in
systems.hpp and also as a base for some future refactoring for common
use cases to access firmware information.

Tested:
- Verified RedfishServiceValidator.py shows no new errors
VERBO - ComputerSystem.v1_0_0.ComputerSystem:BiosVersion
VERBO -         value: IBM-witherspoon-OP9-v2.0.10-2.22 <class 'str'>
VERBO -         has Type: Edm.String Edm.String
VERBO -         is Optional
VERBO -         permission OData.Permission/Read
VERBO -         Success

- BiosVersion now correctly returned in data
$ curl -k -H "X-Auth-Token: $TOKEN" -X GET https://${BMC_IP}/redfish/v1/Systems/system
{
  "@odata.context": "/redfish/v1/$metadata#ComputerSystem.ComputerSystem",
  "@odata.id": "/redfish/v1/Systems/system",
  "@odata.type": "#ComputerSystem.v1_5_1.ComputerSystem",
  "Actions": {
    "#ComputerSystem.Reset": {
      "ResetType@Redfish.AllowableValues": [
        "On",
        "ForceOff",
        "GracefulRestart",
        "GracefulShutdown"
      ],
      "target": "/redfish/v1/Systems/system/Actions/ComputerSystem.Reset"
    }
  },
  "BiosVersion": "IBM-witherspoon-OP9-v2.0.10-2.22",
...
}

Change-Id: I2d7792c724f88aa13bd7b40c0d7f70dd4aa1c807
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>