Adding BIOS attribute to select OS load source

- Previously user had to do this configuration of selecting load
source, alternate load source and DASD load source manually,
and this was tedious task. These are network adapters connected
to PCI slots whose part location information is available in redfish.
- Adding bios attribute to handle selection of PCI slot location codes
for these network adapters to be selected from the GUI webpage, GUI
would then populate these BIOS attributes through redfish and this
selection will be communicated to Host via these PLDM BIOS attributes.

Change-Id: I7fa7cefe3323f4b21fb0335d0292ee136424fbe9
Signed-off-by: Kamalkumar Patel <kamalkumar.patel@ibm.com>