Added BOOTP VendorID -> FileName Mapping

[tantalum7]

Added optional support for mapping BOOTP requests from specific vendorIDs to specific filenames. This is configured by adding a [bootp_mapping] section, and entries for each mapping in the form: vendor_id = filename

For example, to support the multi-stage AM335x BOOTP process: [boot_mapping] AM335x ROM = u-boot-spl-restore.bin AM335x SPL U-BOOT = u-boot-restore.img

If no mapping entry is found for a VendorID, then the filename specified in the [bootp] section is used.